If you still haven't experienced the joys of shooting the undead with friends, the latest Steam sale is an awesome excuse. For the duration of the sale, Left 4 Dead and Left 4 Dead 2 can be purchased for a mere $6.79 each, or bundled together at an even deeper discount of $10.19. And of course, you get all of the delicious DLC with the PC and Mac versions for no extra charge, including 'The Sacrifice.'
Those playing the game on the Xbox 360 are also able to download 'The Sacrifice,' but it's 560 Microsoft Points ($7). In fact, when compared to the current Steam sale, it's more expensive to buy all of the console-based series' DLC than it is to purchase both full games (which include all DLC for free) for either PC or Mac.