Mar 16:17 And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;

Mar 16:18 They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover.

Mar 16:19 So then after the Lord had spoken unto them, he was received up into heaven, and sat on the right hand of God.

Mar 16:20 And they went forth, and preached every where, the Lord working with them, and confirming the word with signs following. Amen.

But still, Jesus said that we'd do greater things than He did, if we pray in His name, we'd get it. If I look around in my church, why do I not see people speak in tongues? Why do I not see the sick healed? All miracles which I think should still happen, but often don't. – 2pietjuh2

I still believe Mark 16 applies today. I think to say miracles have passed away is a cop out. Christ compels us to go into the world and perform deeds which are miraculous. I'm sorry you have not seen many valid miracles. I wish I could provide a list for you of great and wondrous things God has done which have been validated by science over the course of the modern technological era however such is beyond anyone's ability because of the vast amount of research needed to do before and after the miracle and follow up required as science advances to know if it was a happening of God or nature.

Instead I entreat you to examine the power of faith alone from a scientific perspective. I find that the power of faith is documented in the placebo effect. Which is able to remedy all manner of diseases in a way in which science does not understand and even has trouble surpassing.

(LDS)

I can understand your frustration. I want to touch on one point, which I strongly believe in. I know from personal experience that miracles still happen, and I also know that there are living prophets on the earth today, who warn us, prophecy, and perform miracles.

What is a miracle?

“A miracle is an extraordinary event caused by the power of God. Miracles are an important element in the work of Jesus Christ. They include healings, restoring the dead to life, and resurrection. Miracles are a part of the gospel of Jesus Christ. Faith is necessary in order for miracles to be manifested” (Guide to the Scriptures, 165).

One example I would like to give is this: In 1995, a Proclamation to the World was issued about The Family (https://www.lds.org/topics/family-proclamation). In this Proclamation, what a family is in the eyes of God was outlined, what the role of husbands and wives, are and many other points were touched upon in this one page document.

Now fast forward to 2008, this is when Prop 8 was proposed in California, challenging the traditional, and correct view of marriage. Marriage is between a man and a woman. All of a sudden, this document was seen as a very high importance because of the attack on the family. When it was given, it was a good reminder, but not critical.

As for another example with Family, in 1915, the prophet Joseph F. Smith gave the instruction to hold weekly Family Meetings called Family Home Evenings where families are to gather in prayer, song, and a gospel discussion. When this was issued, once again, the families were strong, now however, this is very critical and has been reiterated by more recent living prophets.

Miracles still happen. The world is a wicked place, and often these miracles are attributed to science. I see advancements in medicine, science, space exploration, energy, etc. as miracles. Why? Because God inspires man to invent, He gives them ideas and they run with it.

Another point of interest is when you said this:

My struggle would be a lot easier if I watched the Red Sea split in two. You see?

I can understand your want to see miracles as they were recorded in the OT. One thing I have learned through experience is Miracles do not produce faith, nor do they convert. Belief and Faith will open your eyes to the miracles which surround you. Don't give up hope.

Note: I apologize if this came across as "pastoral advice" or not inline with stack exchange, this is something I am passionate and believe in quite a bit. :)

Paul addressed this issue when he wrote, "For Jews request a sign, and Greeks seek after wisdom" (1 Corinthians 1:22). In the past God indicated that His prophets would do signs and wonders at His direction; if the prophesied act did not come to pass, then they knew it was not of God.

So when Jesus did miracles before the Jews, He did specific signs that God had prophesied through His prophets regarding the kingdom of God. His were not random signs, but ones that showed that the kingdom was there, such as healing the sick and maimed, casting out demons, and raising the dead -- all were indications of the kingdom of God.

Outside of the Hebrew culture, where God worked few or no miracles of the same caliber, signs and miracles wouldn't have the same import or necessarily point to the coming kingdom. The Gentiles have no context in which to place miracles like the ancient Jews did. So Paul preached "wisdom" to them; he "reasoned" with them. We see this throughout the book of Acts as he goes from place to place to argue from the scriptures of the resurrection of Jesus from the dead.

I think this is why we see so few miracles in the modern world (except for within certain church cultures in which God's moving among them is expected).

Because my answer was already turning into an epistle, I will restrict myself to only answering the title question. I am answering from a Charismatic Christian perspective:

Where have all the miracles gone?

Contrary to the premise of your question, there are more miracles occurring in the world today than at any other time in history: The blind see, the deaf hear, the lame walk and the dead are raised - stop your doubting and believe!

At the same time however, and in complete accord with the spirit of your question, there is a seemingly paradoxical hardening in the hearts of men that they willfully ignore the many evidences of God's grace at work if only they would seek for them. This is all according to the scriptures:

The latter glory of this house shall be greater than the former... (Haggai 2:9a ESV)

Truly, truly, I say to you, whoever believes in me will also do the works that I do; and greater works than these will he do, because I am going to the Father. (John 14:12 ESV)

And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover. (Mark 16:17-18 ESV)

but

...when the Son of Man comes, will he find faith on earth? (Luke 18:8b ESV)

But understand this, that in the last days there will come times of difficulty. For people will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, heartless, unappeasable, slanderous, without self-control, brutal, not loving good, treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God, having the appearance of godliness, but denying its power. Avoid such people. (2 Timothy 3:1-5 ESV)

...You do not have because you do not ask God. When you ask, you do not receive, because you ask with wrong motives... (James 4:2-3 NIV)

Many people either don't look for - or upon chance encounters, willfully downplay or ignore - miraculous signs because they're not actually interested in encountering the living God. I wholeheartedly agree with the excellent answers from both Mike and Caleb - and happily concede that a sign is only useful in respect of what it points to. But there are many signs that do point effectually to the risen Lord Jesus Christ - even if you ignore them (the signs) do not ignore Him! Heed well the warning implicit in Mike's 7th point:

As it is said, “Today, if you hear his voice, do not harden your hearts as in the rebellion.” For who were those who heard and yet rebelled? Was it not all those who left Egypt led by Moses? And with whom was he provoked for forty years? Was it not with those who sinned, whose bodies fell in the wilderness? And to whom did he swear that they would not enter his rest, but to those who were disobedient? So we see that they were unable to enter because of unbelief. (Hebrews 3:15-19 ESV)

Why didn't Jesus give a sign?

Jesus certainly performed many miracles, and those were one witness to the veracity of his teaching. (e.g. Jn 5:36) There were also times when Jesus didn't perform miracles, or did them in an unusual way.

Matthew 12:38-43 (NASB)
Then some of the scribes and Pharisees said to Him, “Teacher, we want to see a sign from You.” But He answered and said to them, “An evil and adulterous generation craves for a sign; and yet no sign will be given to it but the sign of Jonah the prophet;…”

There were times when belief was expected before a miracle was done.

Matthew 13:58 (NASB)
And He did not do many miracles there because of their unbelief.

For others, the miracle was done in the way they believed Jesus would/could.

Matthew 9:27-31 (NASB)
When He entered the house, the blind men came up to Him, and Jesus said to them, “Do you believe that I am able to do this?” They said to Him, “Yes, Lord.” Then He touched their eyes, saying, “It shall be done to you according to your faith.”

Matthew 8:1-13 (NASB)
Jesus said to him, “I will come and heal him.” But the centurion said, “Lord, I am not worthy for You to come under my roof, but just say the word, and my servant will be healed.” …
And Jesus said to the centurion, “Go; it shall be done for you as you have believed.”

We see Jesus working with people where they are, even in their imperfect belief.

Mark 9:14-29 (NASB)
“But if You can do anything, take pity on us and help us!” And Jesus said to him, “ ‘If You can?’ All things are possible to him who believes.” Immediately the boy’s father cried out and said, “I do believe; help my unbelief.”

You could also argue that Jesus isn’t obliged to perform miracles on people who wouldn’t believe anyway. His refusal to the Pharisees could fit this situation. Another is found in a parable.

Luke 16:19-31 (NASB)
“If they do not listen to Moses and the Prophets, they will not be persuaded even if someone rises from the dead.”

Why do we not see signs like those now?

There is not a uniform answer among Christians. Some will say that there certainly are. I haven’t personally seen evidence of such things. It would certainly be difficult to prove that miracles cannot happen today. If they can happen, why don’t I see them?

Perhaps our faith is too small (cf. Jas 1:5-8, Lk 17:6) If we don’t (really) believe God could or would do something, should we expect him to do it? If we make ourselves his adversaries, will he show himself to us to prove us wrong?

Perhaps it wouldn’t (really) help us believe. God knows what’s best for us, even when we don’t. His answers to our prayers are not always “yes.”

God’s answers are not always big and loud. (cf. 1 Ki 19:9-12)

Perhaps God is doing something in this age. There was a period of 400 years between Malachi and Jesus. There are some who claim that there were no prophets during that time. Why not? It wasn’t the end of prophecy, surely (since more came later). In that case, God had something planned. He was making things ready in the way he knew was best. Perhaps this is a “quiet” age for some of these things.

Perhaps we haven’t asked or are afraid to ask. I say this about myself as much as anyone else.

Sometimes when promoting our own particular views common sense observations are undervalued at the expense of our own bias. I like too thinking about this topic from a common sense perspective and then arranging my own particular view within that. First, it is quite obvious that the massive spikes of miracles in the Bible occurred at the foundation of the Old and New Covenants, making their authority attested. Under Moses many miracles were performed in killing being a representative of the law and condemnation under it. Under the foundation of the gospel by Christ and the Apostles many miracles were performed giving life, representation the salvation that the gospel provides and the deliverance from the condemnation of the law.

We see the order of things described here with regard to the new covenant:

We must pay the most careful attention, therefore, to what we have heard, so that we do not drift away. For since the message spoken through angels was binding, and every violation and disobedience received its just punishment, how shall we escape if we ignore so great a salvation? This salvation, which was first announced by the Lord, was confirmed to us by those who heard him. God also testified to it by signs, wonders and various miracles, and by gifts of the Holy Spirit distributed according to his will. (Hebrews 2:1-4, NIV)

Here we have the order. Salvation was first announced (i.e. the gospel), then God performed miracles to confirm and testify his approval of this new doctrine that was preached. More directly God's testimony did not only affirm the truth of what Christ and the Apostles preached, but was an unusual and heavenly vindication and seal of approval on their very persons and ministry as infallible guides on establishing the remaining canon of scriptures (so long as they remained teaching the same doctrines to the end). Once these scriptures were completed, they became the final authority in all matters of church practice and Christian living. Second, the only reason why we believe miracles to have occurred is because the scriptures say so.

Third, although Jesus is ‘the same, yesterday, today and forever’, we find God never continuously performed miracles anywhere in the Bible, but over long periods there was an absence of extraordinary physical miracles beyond that of prophecy. Even with the fulfilled prophecies many years pass at point in scripture where nothing new is fulfilled.

Fourth, as a matter of fact miracles do not cause people to believe in God permanently (eg. Pharaoh). They do allow for mammoth shifts in Salvation history, taking people 'by storm' as it were. They do overwhelm peoples minds to make them search for an explanation like concluded Jesus did what he did by the power of Satan, but they do not directly lead to faith, God’s word alone does that by his Spirit.

Fifth expecting miracles in order to have faith is in accordance with the Jews own wicked expectation of miraculous signs (Matth 16:4, 1 Cor 1:22) This may have partly been due to the history of miracles that God performed in establishing the Old Covenant as we have said.

Sixth, the parable of the rich man in hell talking to Abraham was one way in which Jesus tried to expose foolish human thinking that miracles were needed for faith:

“He said to him, ‘If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead. ’” (Luke 16:31, NIV)

A wicked expectation/demand of miracles seems mostly due to a want of belief in the power of scripture and the Holy Spirit's ability to convict men of sin and bring them into salvation thereby.

Why do miracles 'confirm' God's word if they are not needed to arrive at faith? When considering how difficult it would be for a person without God's power to arrest the attention of a large body of people and introduce to them a whole new practice of worship and doctrine, we can see the miracles under Moses and Christ made this possible. Even Pharaoh had to stop and give attention to Moses and question his own beliefs and decisions. Also in the life of Christ, his popularity helped spread his fame. Miracles simply overwhelm human opposition so that even those who hate God and who would never believe, are yet arrested, even if for a moment to allow God's changing of the course of history and establishing of his kingdom as he sees fit.

Seventh, overwhelming miracles like under Moses or during the time of Christ were never meant to be God's regular mode of operation, otherwise they would be, for God is omnipotent and could do it as easily today and he once did before. To say that the people in those generations were more deserving of miracles due to their faith would be to conclude the most carnal opinion possible. These generations who were greatly favored by these miracles were also generations of the greatest wickedness. Those under Moses were destroyed in the desert as utter apostates and that generation under Christ killed the Holy One of God.

Note: In firmly arguing these common sense observations I am not implying that all external extraordinary miracles have forever ceased, only that if they did, it would not matter in terms of our faith and eternal salvation. I am also saying that it is not reasonable to demand them from those who we think have great faith or gifts of the Spirit, or claim to.