A talented L4D 2 modder named Matthew Lourdelet has been hard at work creating a custom community campaign for the game called Cold Stream, and Valve wants your help turning it into official DLC for PC, Mac, and the Xbox 360.
The official Left 4 Dead blog spells out the details of the upcoming DLC, which will include the remaining three levels from the first game (No Mercy was already released in The Sacrifice DLC pack), plus Cold Stream. The Left 4 Dead 2 community get help move the process along, by testing the new level (PC players only), and by making suggestions for what the level should include in this dedicated forum. Once the DLC pack is ready, with will be released for the PC, Mac, and Xbox 360. If your suggestion for the new add-on is that it should be free for Xbox 360, don't bother — people have already posted the suggestion, and thanks to Microsoft's policies for handling DLC, it probably ain't happening.