Turtle Rock Studios or Valve?
– The development studio creating Left 4 Dead was formerly known as Turtle Rock Studios, which was a separate company founded by Michael Booth in March of 2002. Turtle Rock Studios’ work includes a large number of the Counter-Strike Source maps, along with Counter-Strike: Condition Zero and the Xbox version of Counter-Strike. They also are the creators of the CS Bot. On January 10th, 2008, Valve acquired Turtle Rock Studios, absorbing the company.