As revealed on the Left 4 Dead blog, Cold Stream began life as a fan-made campaign. Valve was so impressed by the work of Matthew Lourdelet that the developer decided to release it as an official DLC package on Xbox 360, PC, and Mac.

There is just one little problem though. Cold Stream isn't finished yet. That's where you come in.

If you play Left 4 Dead 2 on PC or Mac, Valve wants your feedback. Keep your eyes peeled for the Cold Stream download, play it, and then offer your constructive opinions over at the Cold Stream Forum .