Left 4 Dead is a co-op shooter utilising Valve's Source Engine. It seems that the game is based in a zombie holocaust and looks to be a logical step on from Turtle Rock's already healthy relationship with Valve.
Previously, it put together the Xbox port of the original Counter-Strike, Counter-Strike: Condition Zero, and the Source engine-based version of CS, Counter-Strike: Source. The studio is also responsible for the Official Counter-Strike Bot which in turn helped Turtle Rock devise its own AI systems.
Turtle Rock was founded in March 2002 by former Westwood/EA man Michael Booth and has just over ten staff working at its offices. Previously, the company had confirmed it was working on a new game with Valve that would be distributed over the online delivery system Steam - and presumably Left 4 Dead is it.
"Leveraging It's industry-leading AI technology and years of Counter-Strike development experience, Turtle Rock is building upon the elements that make Counter-Strike successful and applying them to Left 4 dead," commented Game Newell, co-founder and president of Valve. "What Counter-Strike did for multiplayer action games, Left 4 Dead will do for co-op games."