There's this one, too:
http://grouphug.us/
Out of the three of them, I think PostSecret is by far the most artistic and personal.
It just kind of makes me wonder whether society will change as a result of people having access to an anonymous form of confessional or place to vent, or if maybe it's just that the pressures of our lifestyle mandate some form of release, and it just sort of cancels itself out.