The Team Fortress 2 Website reveals the promised Mann vs. Machine update is now live, adding co-op support to Valve's teamplay shooter. The Mann Up! FAQ has details on how this works, and what you get for purchasing their $0.99 "tour of duty" tickets. Here's an overview:

Valve, creators of bestselling game franchises (such as Half-Life, Portal, Left 4 Dead, and Counter-Strike) and leading technologies (such as Steam and Source), announced today that Mann vs. Machine (MvM), a new co-operative game for its free-to-play online action title Team Fortress 2 (TF2), is now available to download for free via Steam on PC and Mac.

MvM lets you and five friends fight a lethal horde of robots on all-new maps, with the opportunity to upgrade abilities and weapons along the way. Band together to survive all of the robot waves in a variety of missions to earn incredible loot and unlock new achievements.