The 'slaps' game. Where you, basically, slap each others hands for fun. There's some rules to it, but I can't be bothered to explain them.
The best thing about this was that it was actually me who introduced it to the school, after being taught it by my uncle. Of course, I didn't get into trouble or anything, because by the time the teachers found out, it was 14th hand news anyway.
EDIT:
Quote Originally Posted by crop
MP3s (not a game but still)
iPods have games on them, though, don't they?