Orange Box- IS a kick ass game, and i don't care what anyone says, it doesn't matter which system you buy it on... Hands down, you NEED to play this game, no question. If you like Shooters, Action, Kick-Ass Story, Dark Humor, Mulitplayer Madness, and just ultimately feeling like you got your money's worth, get this. It could have been priced at $120 dollars, and i would have bought it. Paying $50 for all 5 is a steal...
Dead Rising- A similar "classic" game. The only game that i found Bloody, Funny, and action packed all in one. (And the Adam the Clown boss's ending is just plain disturbing)
Crackdown- As the name suggests, this takes the sandbox demolition of the GTA series, and makes it smoke "Crack" (Bad pun). I would reccomend if you play GTA ONLY for the random destruction...
Halo 3- Yes, this may be out a FEW years on PC, but even if it is, most of the Online buzz will be gone for this game, redering online play in the PC no where near as good as it is now..
Gears of War- Probably one of the more strategic games (moreso than Halo3). This is more on the realistic side, and has a much more dedicated fanbase. It seems people migrated from Halo 3 to Gears Pretty Quickly.
Xbox Live Marketplace Demos- Why is this getting a spot? Because it allows you download a demo for virtually any game that's released. Which allows you to get a taste for new games to see if you should buy it, or just rent it from a friend... It also allows you to test drive some Arcade games, some of which are Exclusive to XBL.