Counter Strike 1.6

Counter-Strike (sometimes differentiated as Counter-Strike 1.6 and shortened to CS) is a tactical first-person shooter video game developed by Valve Corporation which originated from a Half-Life modification by Minh "Gooseman" Le and Jess "Cliffe" Cliffe. The game has been expanded into a series since its original release, which currently includes Counter-Strike: Condition Zero, Counter-Strike: Source, and Counter-Strike: Global Offensive. Counter-Strike pits a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won by either completing the mission objective or eliminating the opposing force.
The game was the most played Half-Life modification in terms of players, according to GameSpy in 2008.[2]
As of August 2011, the Counter-Strike franchise has sold over 25 million units.[3]