RC's Revenge. I think that's what it was called anyway. Some racing game with RC cars that was a buttload of fun.
Played it for a while, got bored, traded it in, bought it again, remembered why I traded it in before, traded it in again.
And I regret it so... I don't think it was even that good, I just loved it anyway.
EDIT:
I've also technically bought Team Fortress 2 three times, once on each platform.
Traded in the PS3 version, but if all the updates come into effect when Steam hits the PSN, I'll buy it back in a heartbeat.