Valve Software

Developed by Valve, Team Fortress 2 is a class-based, multiplayer FPS. Players choose teams and then compete against one another with various winning objectives such as "capture the flag". There are nine specialized classes that the player can select. Players are encouraged to work together as teams to compensate for the weaknesses of different classes. The game has a unique, cartoon style which allows you to enjoy your repeated deaths at the hands of your turncoat friends.

Winner of over 50 Game of the Year awards, Half-Life set new standards for action games when it was released in 1998. Half-Life: Source is a digitally remastered version of the critically acclaimed and best selling PC game, enhanced via Source technology to include physics simulation, enhanced effects, and more.

Half-Life 2: Episode 2 is the latest continuation of the Half-Life story from your friends at Valve. Gameplay is essentially the same as the previous two games with the emphasis on action. There are some large maps which give a nonlinear tint to parts of the game. A new ammunition is available for the gravity gun which allows placement of Strider destroying "sticky bombs".

If you got the first two you know you are getting this one so why fight it?

Portal is a Source-engine puzzle game. Players use a first-person perspective to explore a three-dimensional environment that is broken up into a series of levels. The only device available to the player is a portal-placement gun that is used to solve the puzzles. Players can place up to two portals that can then transport the player (or anything else) through them.Portal is a puzzle-lite game with an emphasis on faster replays and conditional achievements.

Counter-Strike: Condition Zero is a 2004 release for the PC

Half-Life 2 is an FPS from Valve Software, and was one of the most highly anticipated sequels of all time. You return as Gordon Freeman, the strong and silent-type physicist, and you must do battle with an alien race of enslavers to liberate the surviving remainder of humanity on earth.

Day of Defeat: Source is a team-based multiplayer FPS from Valve Software which has players fight during World War II as members of either the Axis or Allies.

Counter-Strike: Source is a multiplayer FPS from Valve Software that pits terrorists and counter-terrorists against each other in a variety of settings to battle it out.

Half-Life 2 is an FPS from Valve Software, and was one of the most highly anticipated sequels of all time. You return as Gordon Freeman, the strong and silent-type physicist, and you must do battle with an alien race of enslavers to liberate the surviving remainder of humanity on earth.
Also applies to Half-Life 2 Lost Coast free downloadable level.

Half-Life is the groundbreaking FPS that follows the "everyman" hero Gordon Freeman. This is the first installment of the ongoing series, which ties together aliens and government conspiracies. Really, this game doesn't need any introduction...