The Facebook (lil) Green Patch application has generated over 94 thousand dollars so far toward rain forest preservation by inviting social networking among friends and strangers over cartoon gardens.

http://apps.facebook.com/causes/hall_of_fame/show/2979

The funds are contributed to Nature Conservancy's Adopt an Acre.

And perhaps the amount of money generated is not even the greatest good such pastimes create. Comaraderie, diminishing the sense of isolation, exposure of people to different cultures via friendly encounters...

As others have noted before, harnessing the energy from having fun has as much, if not more, potential to accomplish good than many well-intentioned, earnest endeavors that feel like obligation.