Some exciting news from Valve! After giving several hints in the past little while, on August 15th an update for the free-to-play Team Fortress 2 will hit steam, introducing a new game mode: Horde.  
This new mode will place 5 users playing co-op against a horde of robotic enemies from the Gray faction. If you're asking "Gray faction? I thought it was just Red and Blu? Clearly you're out of the loop! Head on over to Team Fortress' site for more info!