What do you like to do? Personally I like "wobble". Its a picture app that lets you make parts of the pic bounce when you shake your phone. I cant tell you how many chicks have let me snap pics of their racks (usually clothed ) just to see it work.
Some others I mess with alot are the iheartradio, colorsplash, google earth and mapquest. I got a couple decent games too like topple, space deadbeef and tower madness.