As is their way, Valve has acquired yet another developer specializing in the Source engine. A news article on Valve’s site confirms “the acquisition of Turtle Rock Studios, the Orange County-based development house behind one of 2008's most anticipated games, Left 4 Dead.”  Turtle Rock Studios has contributed several maps for Counter Strike: Source, and is developing Left 4 Dead, a co-op zombie that also uses the Source engine.  None of this is terribly surpising, as Valve has embraced the modder community unlike any other publisher, and has a long history of snatching up small dev teams that make good use of the Source engine.  These include Nuclear Monkey Software, creators of Narbacular Drop (spiritual predecessor of Portal), Minh "Gooseman" Le and Jess Cliffe., creators of Counter-Strike and Robin Walker, John Cook, and Ian Caughley, creators of Team Fortress.  

Here’s some nice looking footage of an early build of Left 4 Dead.