In case you missed the news late yesterday, the Left 4 Dead "Survival Pack" downloadable content is now available both for PC and Xbox 360 users. Valve offers the pack for free on both Steam and the Xbox Live Marketplace.
This DLC adds a new Survival mode, in which players fight off wave after wave of computer-controlled zombies before their ultimate demise. The goal is to survive as long as possible, and Valve provides team leaderboards to encourage competition. Oh, and the Survival mode comes with a new, exclusive Last Stand map with a lighthouse atop a cliff.
Other than that, the Survival Packs lets players fight each other in the game's Versus mode in two additional campaigns, Dead Air and Death Toll, which were previously available only in co-operative and single-player modes. Valve has also thrown in a number of bug fixes and miscellaneous little tweaks, which Shacknews lists in this news post.
If you haven't grabbed Left 4 Dead yet, the Shack points out that both the PC and Xbox 360 versions are "now available for a special price." The PC version is $39.99 instead of $49.99 on Steam.