Not that you have any shortage of things to play this weekend, but how many of them are free? Co-op zombie slaughter extravaganza Killing Floor begins a free weekend on Steam today.

Which is great timing, because it also gets a free* add-on today. The Level Up Free* Content Pack adds three new maps, all quite good. In fact, I'd say each of the new maps is better than the game's original maps. The insane asylum that was the site of a recent Halloween-themed rave is both a cute concept and timely. The forest with a waterfall is finally another great outdoor map, much creepier than the farm map. There are also new guns, a level cap increase for your skills, and a new skill to take advantage of the new pipe bombs and grenade launchers. Also, be on the look out for a zombie who shoots fireballs, which hardly seems fair.

* It's not quite free. The new content includes a streaming advertisement in the multiplayer game lobby, which seems like a fair trade to me. You give me a bunch of free stuff and I'll gladly suffer a small ad before my match starts.