Well, thanks to all these one use codes for extra content, its unlikely I will trade in nany games anymore. Because as far as I'm concerned, I wouldn't want someone to buy a preowned copy of the game I played and not get the full experience (without having to pay extra)
But I know tats not what the OP is asking.
Games I'll hang onto just for the fun would be Burnout Paradise, Uncharted 2, Bayonetta, Mass Effect 2 (and to a lesser extent 3), Saints Row 3 (But again, thats partly because of the online pass shit), Mirrors Edge, I guess.
Games I'm sorta forced to hang onto because they're worth fuck all to trade in...
Heavenly Sword, Dead Space 2 (how I wish I'd traded that in when it was still worth something), Sacred 2, well, pretty much the rest of them...
Captcha: I like people. Do you Captcha? Well then go and fuck yourself, you cheerful bastard.