Left 4 Dead players: Valve has a big treat for you today. If you haven't guessed what it is yet from that big ol' sign up top, it's the "Crash Course" DLC, an expansion for Campaign, Versus and Survival modes, available for the PC and Xbox 360.

Note the PC version is free, while the console version will set you back 560 MS Points, though this is a decision of Microsoft's, not Valve's (e.g. allowing free DLC on the 360 makes publishers of paid DLC look bad). So much for the Left 4 Dead 2 boycott, then!

 Crash Course

• Left 4 Dead's latest DLC delivers an all-new single-player, multiplayer and co-operative gameplay experience, expanding the game universe with new locations, new dialogue from the original cast, and an explosive finale.

The Story Continues

• At the finale of No Mercy, our four Survivors fought their way to the rooftop of Mercy Hospital, where they were flown away to safety in a helicopter. Crash Course continues the story moments later, taking our heroes from a fiery helicopter crash in the outskirts of Riverside to the escape vehicle that will lead them to the events in Death Toll.

• While containing both Survival maps and a Co-operative Campaign, one of the primary goal of Crash Course is Versus mode. Crash delivers a complete, streamlined Versus mode experience in just 30 minutes. A recharge timer for infected teammates has also been added, so you can better coordinate your attacks. Item spawn behavior has changed for more balanced gameplay.

Included with the DLC are ten new achievements, as well as tweaks for Versus mode and matchmaking -- now all weapons and items will remain identical for the second round of a chapter in Versus, and it will be easier to find and join games. Update: a second patch has been released, containing some minor tweaks.

And to top it all off, Valve is currently selling the game for 50% off ($15) on Steam for the next three days, so if you've been on the fence, it's probably a good idea to grab it before October 2.