Those of you who pre-ordered Valve’s new co-op zombie shooter, Left 4 Dead, from Steam will have probably already played through the demo. For everyone else, however, now is the chance to give it a try as the demo has officially gone public.

Left 4 Dead is Valve’s first major single title since the release of Half-Life 2 and it promises to redefine the co-operative genre, much in the same way that Half-Life and Counter-Strike changed the single and multiplayer action genres. The demo features two full levels from the final game, playable either solo or online, and is available for both PC and Xbox 360 users via Steam and Xbox Live Marketplace respectively.

The final game will be made available on November 18, but those of you who have been enjoying the demo since late last week can let us know your thoughts in the comments. A trailer for the game and detailed system requirements are available at the official website.