This deal is more of a barter than a free game offer, but it can be a great deal if you happen to have, and be finished with, one of the games on their acceptable trade list. You can read in detail how the free game rental offer works on their site, but here is the short version.
You order a free mailer from them and send in your game. For each approved game you send in you get two months of free game rentals. They normally charge $18.95 per month. The way I look at it, you get a lot more value in the two month unlimited free game rentals than you would for trading the game at a video game store or selling it on Ebay.
The free game rental offer is available for PS2, Xbox, and GameCube.
Free Game Rentals at Red Octane