COD4- I wasn't able to connect to xbox live at the time and I'd played through the campaign, and traded it in for nothing really. Now I can connect to live, but have no good live games that I can play with my friends. They all own CoD4 though.
Mass Effect- Traded in with CoD4, got forty bucks for the lot of them, stupidest trade I think I've ever made. Borrowed it from a friend though, so it's kind of okay, getting my fix.
Monster Hunter- Traded it in for Tales of Symphonia, loved Tales, but still, wish I could have had more time with Monster Hunter, didn't really give it a lot of chance and I've been wanting to get it again, but can't find it anywhere.
Betrayal at House on the Hill- A board game. Traded it to a friend for a copy of Saint's Row. Had planned on turning around and buying another one since the board game was only $20 bucks at the time and Saint's Row was still pretty new. Put it off. The game went out of print and now the cheapest one I can find is used, pieces missing, and $125.