Lets see. You have:
-Estarland - great site, seems to give a good amount of money for trade ins, even if you only have the game, no box or manual. Problem here is that, like gamestop, they take away 30% for cash (ie, you only get 70% of the trade in value listed). They also have great deals on games too, though.
- GameTZ - amazing site, loved it (when I use to use it). Great people and you can easily see who has what, what they are willing to trade/buy the game for, feedback, etc. Also has a great chat which you can trade games quick (at night is the best time).
-Trade Games Now - also a good site, mostly good people there, some good deals.
-Goozex - never really got into it, but from what people tell me, its a great site.
Theres some others that I can't think of at the moment, but these are some of the better sites that I can think of.
*And as a side note, Gamestop does actually still give cash, but they take something like 15%-20% (prob. more) off the trade in value.