Valve is currently giving away its superlative zombie shooter, Left 4 Dead 2, for free on download service Steam. To get your copy of the game, you'll need to log in to a Steam account, head to Left 4 Dead 2's store page, and then click "install game." The holiday deal itself expires on December 26th at 10AM PT / 1PM ET, but once the game is downloaded, you'll have access to it forever.
Left 4 Dead 2 is playable on PC, Mac, and Linux. It allows four people to take the role of heavily-armed survivors stranded in a zombie-infested US, and rewards co-operation and co-ordination with hours of tense and tactical combat. Valve's shooter is best when played with three other people, but you've only got 12 hours before the deal is up, so interrupt your friends' holiday celebrations as soon as possible to coerce them into downloading it.