Valve has announced a new gameplay mode for Team Fortress 2 called Mann vs. Machine. MvM lets you and five friends fight together against a horde of robots on new maps. "Band together to survive all of the robot waves in a variety of missions to earn incredible loot and unlock new achievements."
Mann vs. Machine will be released on August 15, 2012.
As always, Team Fortress 2 is free-to-play and better with friends, so get playing!