score:2
Adioso lets you search for any time. Additionally Skyscanner lets you search across an entire month. Generally Skyscanner has a better range.
Bren on the road has a good overview of searching for a trip in this way.
Upvote:1
Where you are from and where you want to go? For flights around Europe you could try airixo.com. It will show you all the places you can fly to from your location, when and for how much (you can filter the results by maximum number of changes etc.).
Downsides: it's basically just Europe and it seems they search only low-cost carriers.