What's your favorite facebook app.?

I like Pet Society. Same idea as Yoville I guess except you have a pet that you "design" yourself, gave it a name and visit and do things with all your friends that have the same application. This earns you coins. You can run around the town and earn more coins in the trees (money does grow on trees lol). The coins allow you buy things at the shops like clothing, food, furniture for your home, wallpaper, flooring, luxury items etc. Every day you visit, there is a daily lottery to get more free coins. You also earn points that give you additional fun things like balls to play with, a bigger house, whatever. I am quite addicted to it.