SlickDeals reports that "Gamestop offers a Free Used Game withl your purchase of 2 used games (with code GSB2G1). The lowest price game will be the free one. This is valid in store and online both."
Another thing: if you like the idea of having a video game system but haven't wanted to spend the money, GameStop is also offering refurbished Nintendo GameCube systems for $30! You can get used games for under $10 and, with the coupon code, walk out with a GameCube and three games for less than one Ulysses S. Grant.
Buy 2, get 1 free on used video games at GameStop