Team Fortress 2. The update will award three new weapons to both the Soldier and Demoman classes -- weapons which will be revealed piecemeal over the course of the next week. However, the two classes will be fighting during the week to unlock a secret fourth weapon for their profession. Whichever class can kill the other the most will be victorious, and can claim the additional armament.
You can stay on top of the current kill count on Valve's hub page for the update, which is appropriately titled "The War." Our money's on the Demoman, but that might just be because we love his Shrek-inspired accent.