Turtle Rock Studios have released a "lost" Left 4 Dead campaign that never saw the light of day. Dam It is an incomplete campaign in almost every way but it is still playable.
Just finished putting the Dam campaign that we streamed a while back into a L4D addon. It's gray box, and some code features are missing but it's playable from beginning to end. It also includes two standalone survival maps that never shipped.

"Gray box" means that it's largely untextured and obviously incomplete. The campaign bridges the gap between the Dead Air campaign and the Blood Harvest campaign.

You can download Dam It and try it for yourself right here. An overview of the campaign layout can be seen below.