Free on itunes.apple.com
Remember this game? It was probably on your very first cell phone, back before it had a touch screen and tons of apps. If you've never actually played it before, all you have to do is tell your snake to go in a certain direction. The goal is to eat up as many little dots as you can to make your snake grow bigger. Of course, you can't hit into the wall or into yourself, or you'll die and the game is over.