Killing Floor has come a long way since its days as a relatively awkward Unreal Tourament 2k4 mod. Rather than flare out and fade into obscurity like so many indie multiplayer shooters, it has risen again and again, becoming one of Valve’s own favorite holiday fixtures, and was even the focus of a great amount of attention during the Portal 2 ‘Potato Pack’ event. Tripwire Interactive are nothing if not giving, either. December is here, and with it a free weekend for the game, starting tomorrow. On top of that, a horde of Christmas-themed monsters to slay (sleigh?), a boatload of new weapons to play with and even a new level and all for the price of free.