Tetris. It was by accident too. I didn't even have my iPhone yet. So then I had to get one or it would have been a waste of $10.
My first purchase was Super Monkey Ball - I bought it the day the app store launched I paid $10 for that piece of crap
I bought all these for $10, RS2009, Hero of Sparta, Super Monkey Ball, Cro Mag, Bejeweled, Bugdom 2, All of Pangea Software games for $10 (), Asphalt 4, Guitar Rock Tour, Aqua Forest, and a lot more