An in-development community campaign (by Matthew Lourdelet) is set to be released as DLC for L4D2 (PC, Mac, X360). Unlike the previous two DLC packs for L4D2, Cold Stream will not be story-heavy. The three campaigns from L4D1 will also be included in the pack.

No solid release date, aside from "sometime after Portal 2". The community is also invited to help test the community campaign.

"He recently shared it with us and we love it so far but it isn’t done. We were tempted to have him quietly finish it with our help and release it as part of an upcoming DLC.

Tempted… but we decided against that. Instead, we think we have a better idea. We want you the community to be part of the process of taking his unfinished campaign and releasing it as DLC on the Xbox 360, PC and Mac."

"In the next few weeks we will add Cold Stream to the UI (user interface) and we will all begin testing it. As we receive feedback and test data we will also share some data we collect on player experience and show how we put that data to use. We will include you with regular posts, polls, and updates.

We have created a special forum just for feedback on the campaign and this project (Cold Stream Forum). While the beta will only be available for PC and Mac users, we appreciate feedback and ideas from the entire community. This is an experiment for us, so please let us know what you think and what you would like to see in the DLC – just remember the constraints."

From the official L4D blog. promoted forum item