Christmas promotion madness. Arena-based co-op zombie shooter Killing Floor is adding a Pyro character for anyone who also owns TF2, and TF2 is adding a new gas mask for the Pyro for anyone who owns Killing Floor - or buys it before January the 4th. It's part of Killing Floor's Twisted Christmas festivities, which also add a Santa's Evil Lair map, a Baddest Santa character skin, and Christmas-themed zombies including undead reindeer.
Killing Floor is an office favourite at PC Gamer, and this is an impressive amount of weaponised festive cheer. See their Christmas update page for the rest of the disgusting details.