Though the news is not yet reflected on the Steam Website, Valve announces the Windows editions of The Sacrifice DLC for Left 4 Dead and Left 4 Dead 2 are both now automatically available through Steam, free of charge for owners of those games. This is accompanied by the release of the Mac edition of Left 4 Dead 2, which, like all Steamplay games, is free for those who already own the Windows edition. For those who don't already possess the zombie shooter sequel, Left 4 Dead 2 is currently on sale for 66% off, as is the original Left 4 Dead (though the Mac edition of that is not expected until the end of this month). The Xbox 360 editions of both DLC are expected later today. Here's word on The Sacrifice for those who have only taken a passing interest to this point:

"The Sacrifice" is the prequel to "The Passing," and takes place from the L4D Survivors' perspective as they make their way South. In addition to advancing the story, "The Sacrifice" introduces a new style finale featuring "Sacrificial Gameplay" where players get to decide who will give their life so the others may live.

The Sacrifice DLC for L4D1 is a separate download than The Sacrifice for L4D2.

In The Sacrifice for Left 4 Dead 1 gamers receive the complete Sacrifice campaign with maps playable in Campaign, Versus, and Survival.

In The Sacrifice for Left 4 Dead 2 owners receive "The Sacrifice" campaign playable with the Left 4 Dead Survivors. In addition, those with Left 4 Dead 2 receive L4D's "No Mercy" campaign. Both campaigns are playable in Campaign, Versus, Survival, and Scavenge modes and both will feature the Left 4 Dead 2 Special Infected, items, and weapons. Yes, Zoey with a chainsaw!