GameStop has really good prices on used games for all systems. And if you have a GameStop card (you get it when you buy a subscription to Game Informer, 10 issues, $10), you get 10% off of all used stuff (games and accessories).

They even have old school stuff like Nintendo, SNES, Sega, and Dreamcast.
Blockbuster also has some older games for pretty cheap.
I'll have to remember that Circuit City sale, thanks for posting. Too bad they don't have Fatal Frame for the PS2 on there...