GWAP (Games with a Purpose) hopes to save the world (or at least make it easier to find out information about saving the world) through people playing games for free.
GWAP pairs individuals in games against each other to improve search algorithms. For example, in Tag a Tune, you try to figure out if you and an opponent are listening to the same song by trying to describe it (and in the process help search methods for MP3 files). Or the ESP Game shows you and a partner (in different locations) the same image and you try to guess what words your partner is using to describe this image.
Now, if only they can find a way to use game playing to increase social trust and collaboration outside of games…
We reported on other efforts to use the public to do tasks of public good (like Mechanical Turk to map the surface of Mars in this blog post) or here or here.