Valve announced today that new content will be coming for the PC and Xbox 360 versions of Left 4 Dead this spring.
The first L4D downloadable content will be called the L4D Survival Pack. It will introduce a new multiplayer game mode called Survival, as well as two complete campaigns for Versus Mode.
In addition, Valve will also release the first Left 4 Dead Source Software Development Kit (Source SDK). The Source SDK will allow PC gamers and aspiring developers to create custom Left 4 Dead campaigns that will be discoverable via Left 4 Dead's matchmaking system.
The SDK release is also due for release this spring, and will be free of charge to all owners of L4D on the PC.
Last, but not least, a Critic's Choice Edition of the game will be available in retail stores this spring, and will include access to all the content introduced in the L4D: Survival Pack.
Read Valve's complete press release here