Of whom is God jealous?

This is actually an interesting question. "Why the heck is God jealous of other idols?" is the question you are asking, when really you should be thinking outside the box.

The real question you should be asking is whether or not the writers mistranslated.

In the King James Bible as published by the LDS Church, there are footnotes on the bottom of each page that attach to various words in each of the verses. One of the footnotes in Exodus 20:5 is attached to the word "Jealous" and gives the explanation of "HEB qannah, 'possessing sensitive and deep feelings.' This word was either supposedly the word from which the interpretaion of Jealousy was derived from when translated, or the word "jealousy" had an alternate definition that, back when the KJV was being created, was akin to the meaning of the Hebrew word "gannah." If that is the case, then if you use that word instead of the word "jealousy" we receive a completely different interpretation of the passage.

Instead of: "Thou shalt anot make unto thee any graven cimage, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth: Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;" (Exodus 20:4-5)

...which seems to display a harsher nature in God than the New Testament would seem to indicate, we have this:

"Thou shalt anot make unto thee any graven cimage, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth: Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God [am a God with deep and sensitive feelings], visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;"

With this translation, we gain an insight into the sensitive nature of our Heavenly Father. He isn't wrathful and jealous when we worship other idols; he is hurt. He is sensitive, he cares about our feelings intimately, and when we don't try to share that same sensitivities, it hurts him to the core.

He wants us to have a relationship with Him, and when we turn our backs on Him, to worship and give our attention to things that don't even matter, it hurts our God to the core. His own children, the people that he created lovingly and tenderly with his own hands, are denying his existence and giving more respect and honor to an inanimate object than their own Father! To add insult to injury, the idol worshippers attribute that inanimate statue with doing the very thing that He Himself has done!

I imagine that's got to sting quite a bit: To have the children he loves so much, completely and willfully deny that he even created them. If you had sensitive feelings, wouldn't you be angry too?

Imagine you have a child that grumbles anytime you send him on errand but gladly respond to strangers ready to do anything for them.

Imagine having a wife/girlfriend who is always complaining when with you no matter how you pamper her but feel more comfortable and excited being with others even in your presence.

We are God's property, he owns us. He has invested greatly in our past, present and future. And who would make such investment and see another person receive the praises for what they did not do.

Remember Aaron when he made the golden calf? God was very angry and jealous because they gave the glory that belongs to him to the golden calf - something that is far inferior to him.

So also is God jealous when we take sides with Satan, giving him (and his idols) glory, praises etc for what they actually didn't do.

It hurts so much when someone you love doesn't give you attention and gives someone who is inferior to you the attention you deserve or worked for.

So, it is not about who, it is more about why?

I don't know about you, but I have seen people get engrossed in things and start excluding their significant others. I know one man who was an alcoholic and another addicted to computer games. In both cases, their significant others (one was married, the other dating) grew upset over this situation — they felt excluded and were jealous of the others on the computer or at the bar. These men were monogamous, but they still incurred jealousy.

From seeing this human jealousy, I can see how the Divine, who has no real rival, can claim a form of "jealousy" when people reduce His role to something trivial.

According to Paul, God is jealous of His own creation:

For what can be known about God is plain to them, because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things.

Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.—Romans 1:19-25 (ESV)

"Images" references the idolatry found in the Ancient Near East and condemned in Exodus 20 and elsewhere in the Old Testament. Jesus expanded the definition of idolatry to include anything that demands the place of God, which isn't God:

No one can serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other.—Matthew 6:24a (ESV)

The immediate reference, of course, is money, but other worldly things can take God's place as our master:

And others are the ones sown among thorns. They are those who hear the word, but the cares of the world and the deceitfulness of riches and the desires for other things enter in and choke the word, and it proves unfruitful.—Mark 4:18-19 (ESV)

Conclusion

God is so amazing that He is able to create things that seem worthy of worship. But those things are not worth our time, energy and devotion compared to the surpassing glory of God Himself. God wants us to spend our lives on the one thing that is more valuable than ourselves: Him.

To illustrate what this might mean, consider the career of Blaise Pascal, mathematician, physicist, inventor, writer, philosopher, and all-around genius. Around the time of his conversion, Pascal wrote a poem called "Fire" that begins:

GOD of Abraham, GOD of Isaac, GOD of Jacob
not of the philosophers and of the learned.
Certitude. Certitude. Feeling. Joy. Peace.
GOD of Jesus Christ.
Deum meum et Deum vestrum.
Your GOD will be my God.
Forgetfulness of the world and of everything, except GOD.

He gave up everything (mostly and most of the time) to focus his thoughts on God—a task that remained unfinished at death. While he was, perhaps, too extreme in his asceticism, I do not believe that he now regrets leaving the world behind him. Personally, I consider Pascal one of my heroes of the faith.