Vermintide is free on Steam. What a glorious day!
Warhammer: End Times - Vermintide is basically Left 4 Dead except with rats instead of zombies and with persistent progression for each of the characters.
Now that a number of free updates have sorted out its loot problem, it's a heck of a good time if you have even one pal to watch your back. Would you like to try Vermintide? Now you can.
Head on over to Steam to try Vermintide for free. If you enjoy yourself, you can purchase it at 50% off - or consider the newly-released console versions on PS4 and Xbox One, perhaps.
Vermintide is set in Warhammer: End Times, the last hurrah of the classic Warhammer fantasy universe before Games Workshop rebooted the whole thing. There aren't many games I can think of where you're playing in the middle of an apocalypse you can't actually avert. Knowing this gives the whole thing an interesting flavour, I think.