On August 15th (that's two days from now, if you're not near a calendar), mortal enemies Red and Blu will unite against a common robotic Gray foe. Valve announced today that Team Fortress 2 is getting a horde mode in which both teams will make a last stand against waves of bomb-carrying mechanical enemies, up to and including giganto versions. We don't yet know specifics, but we'll be getting new maps, multiple objectives, and some type of ability upgrades as a match goes on, and there's a promise of "incredible loot" to be earned. And, as is TF2's custom, it'll all be free.
Source: Gamespy