Turtle Rock Studios must be cleaning out some old closets now that they're done with Evolve, because three days ago the team released the final, unfinished Left 4 Dead campaign they'd been working on before Valve took over the zombie shooter.
The campaign is called Dam It, and begins at a small rural airfield (the C-130 you escape on at the end of Dead Air has landed, then crashed into some buildings). Players must then fight their way through an apple orchard, ravine, hiking trail and camping ground, before ending with two finales, one at a logging camp (where players are actually encouraged to split up and provide covering fire) and the main one at a hydroelectric dam.
Here's what it looks like in action, courtesy of a playthrough by Bolloxed (so no, that is not my YT thumbnail):
It also includes two standalone survival maps that never shipped.
To recap, the original Left 4 Dead was created by Turtle Rock before the game (and company) were bought by Valve. Turtle Rock were later offloaded while Valve worked on Left 4 Dead 2 internally, and went first to THQ, then 2K with its pitch for Evolve.