I kept Terminator Salvation. Not sure why...I guess it's just one of those "So bad it's good" kind of games, except it's not good. I dunno, just having it is like a bit of a novelty in and of itself. I can pop it in and say to my friends "Hey, you wanna see a REALLY crappy game?"
I still have Killzone 3 too though, as I do intend to play it again and see if maybe it sucks less on the second playthrough, but I won't be touching multiplayer and I really don;t have the desire to play the Campaign again, so it just sits there.
Usually though, if I don't like it, I get rid of it.
Mafia 2 because I was waiting for a deep discount on dlc but could care less now. State of Emergency because my dog doesn't have a frisbee. oh and WWF Warzone because Ahmed Johnson. Eat a reef and stay punch!