As promised, Valve today released the downloadable "Crash Course" campaign for the PC and Xbox 360 editions of its cooperative zombie shooter Left 4 Dead.

The expansion, free for PC players and 560 MS Points ($7) for those on Xbox 360, delivers a new two-chapter campaign playable in single-player, co-op and competitive multiplayer. The download also adds several new maps for use in Survival mode.

In celebration of the DLC's release, Valve is offering 50% off the PC edition of the game--bringing it down to $14.99--until Saturday, October 2. The company has previously said that it plans to keep supporting the original Left 4 Dead even after Left 4 Dead 2 arrives on November 17.

"The primary goal of 'Crash' is to deliver a complete Versus mode experience in just 30 minutes, resulting in a streamlined version of the game's existing Versus campaigns," Valve explained, with patch notes for the PC version just below:

Updates to Left 4 Dead have been released. The updates will be applied automatically when your Steam client is restarted. The major changes include:

• A new campaign, "Crash Course", is now available for play in Campaign, Versus and Survival modes
• In Versus mode, all weapons, pills, health packs, molotovs, pipe bombs and other items will now be identical for the second round of a chapter
• Matchmaking has had several improvements to make finding and joining games even faster
• Ten all new "Crash Course" achievements have been added