Halo 3:ODST is technically an expansion to Halo 3. You play as an Orbital Drop Shock Trooper, not a UNSC Spartan like you did in Halo 1 - Halo 3. It looks really cool from the videos I've seen. Here's a gameplay trailer: http://www.gametrailers.com/video/e3...o-3-odst/50010 -- I guess there's a "Firefight" mode which is essentially like Gears of War 2's Horde mode. Since I haven't played GoW2 that much I don't know what that means.

Halo: Reach was just announced at E3. There's not too much known about it yet although bungie.net has a trailer. http://www.bungie.net/Projects/Reach/default.aspx