Developer Fatshark's excellent four-player co-operative rat-smasher Warhammer: Vermintide 2 is currently free to play on Steam, and will remain so until Monday, September 3rd.
Vermintide 2, for the uninitiated, plays out a little like Valve's classic Left 4 Dead series, albeit with particularly brutal combat, and a bunch of horrifyingly malevolent rat people to maim instead of zombies.
In Vermintide, teams of four players - choosing from one of five distinct heroes, each offering three possible "career" classes - battle it out in a relentless assault of sword swings, axe plunges, whistling arrows, magic, and bullets. However, careful coordination is required to fend off the Skaven hordes, scour stages for reward-boosting items, and live to see another day.