1. Portal/Orange Box : I think most people who game fairly often have heard of portal before, it's that game where you can shoot blue and orange rings 'portals' and step into one and come out the other to solve various puzzles. Right now, the second portal game just came out and as people are jamming into stores to buy the newest release I would probably be trying to pick up a used copy of the first Portal for cheap.
2. Fallout New Vegas : I played the third Fallout game just before my xbox died, and I have to say it is one of the best games I've ever played. If you've played Oblivion you would immediately recognize the feel of the game, but the setting is completely different, and I liked the nuclear wasteland much more than the mystical Cyrodil. You can get it for under $20, and I think that's a good value for any game let alone a great one like Fallout
3. Forza 3 : If you enjoy racing games you probably already own a Forza title, but if you don't I think you couldn't beat Forza 3 which is now selling for $23 used. The amount of customization you can do even in Forza 2 was unbelievable, and with online multiplayer you can expect to enjoy this for many hours. Plus you get to drive really expensive cars, bonus points if the car you own is in the game.
4. Pokemon Black/White : If you want value gaming and you like making animals fight to the faint, Pokemon is the best game for you. If you don't know Pokemon, get out of here right now, I don't want your kind here. Pokemon is one of the few games I'd buy new because it starts relatively cheap at $35 and each new version instantly becomes one of the greatest games of all time even though they keep introducing new Pokemon that aren't as cool as the old ones. If I could only buy one game on this list it would have to be Pokemon.
This has been another post about video gaming from a person with no Xbox 360. Hopefully I can change that soon.