ACT NOW, SUPPLIES ARE LIMITED!!
Print that coupon out and take it with you. You 'll have to go to GameStop, though. Sorry.
You'll get 25% off the price of any used title for the 360. This may help justify some game purchases (i.e. Dynasty Warriors Gundam is still around the $50 price mark used), or just give you an excuse to go out and buy some stuff you've overlooked.
The coupon can be used for multiple visits, and will apply the rebate to 2 games.
If you have GameStop's reward (EDGE) card, the 10% rebate that the card bestows WILL STACK (to a total of 35%). If you haven't signed up for that yet, it's $15 for a year (comes with a subscription to GameInformer).
ALSO NOTE: GameStop is running a promo that gives you an extra 20% towards trade-ins when applies to GTA4, Mario Kart Wii, or MGS4.
ADDITIONALLY: Their policy allows you to return a game within 7 days of purchase. For something like Army of Two, it's a great way to get it, play through with a buddy, then chuck it for something you actually do want to play.