Microsoft has removed the multiplayer portions of Halo 3 spinoff ODST and posted the game for digital download from its Games on Demand category of the Xbox Live Marketplace.
Halo 3: ODST is actually one of my favorite titles in the Halo franchise. In the game, you play as a member of the elite Orbital Drop Shock Troops (ODST’s) during the assault of Earth shown at the beginning of Halo 3 proper.
Your character is underpowered (he’s no Spartan) and can’t dual wield or regenerate health. It’s an interesting game and deviates quite a bit from the typical Halo experience. It really almost feels as different and fresh as Halo 4 does. I’d say it’s worth your time, and at only $14.99 it won’t break the bank either. (and it’s got Nathan Fillion in it too)
Just keep in mind that there’s no multiplayer component, though it’d probably feel outdated next to Halo 4 and even Reach anyway.