You have from right now (a bit past 2PM ET on December 25) until tomorrow (December 26) at 1PM (ET) to claim your free copy of Left 4 Dead 2 on Steam from Valve. Once you claim it during the next day, you have the game forever. It's yours to keep for free!

To summarize: Left 4 Dead 2. Free. Limited time offer. Merry Christmas!