Valve

Counter-Strike

Counter-Strike (also known as Half-Life: Counter-Strike) is a first-person shooter video game developed by Valve. It was initially developed and released as a Half-Life modification by Minh "Gooseman" Le and Jess Cliffe in 1999, before Le and Cliffe were hired and the game's intellectual property acquired. Wikipedia

Counter-Strike: Source

Counter-Strike: Source is a first-person shooter video game developed by Valve and Turtle Rock Studios. Released in November 2004 for Windows, it is a remake of Counter-Strike (2000) using the Source game engine. As in the original, Counter-Strike: Source pits a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won either by completing an objective (such as detonating a bomb or rescuing hostages) or by eliminating all members of the enemy team. Wikipedia

Day of Defeat: Source

Day of Defeat: Source is a team-based online first-person shooter multiplayer video game developed by Valve. Set in World War II, the game is a remake of Day of Defeat. It was updated from the GoldSrc engine used by its predecessor to the Source engine, and a remake of the game models. The game was released for Microsoft Windows on September 26, 2005, distributed through Valve's online content delivery service Steam. Wikipedia