CS1.6 / CS:GO / CS:SOURCE

Counter-Strike 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. By the fourth beta version, Valve Software, the developer who created Half-Life, began assisting in the development of Counter-Strike.[1] In 2000, Valve bought the rights to Counter-Strike, and would publish the title for Microsoft Windows that year, and later in 2003 for the Xbox. Mac OS X and Linux ports were available in January 2013.

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 terrorists against a team of counter-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
As of August 2011, the Counter-Strike franchise has sold over 25 million units.