Valve wants to say "Merry Christmas". So everyone gets a copy of Left 4 Dead 2. You get a copy, you get a copy, you get a copy, you get a copy.
If you jump on Steam before 10am PT on December 26 (US TIME), you can download the multiplayer shooter for free. The only catch is that you need to install it, not just "purchase" it, and once that deadline has passed, you'll still own the copy.
Left 4 Dead 2 [Steam]