Half Life 2 (including Episode 1 and 2)

Half-Life 2 is the sequel to the highly acclaimed Half-Life, and similarly is a singleplayer science fiction first-person shooter video game and a signature title in the Half-Life series. Developed by Valve Corporation, it was initially released for Windows XP on November 16, 2004, following a protracted five-year, $40 million development cycle, during which a substantial part of the project was leaked and distributed on the internet.
The game was developed alongside Source and Steam. It introduced the popular game engine and, because of Steam, is the first video game to require online product activation.
Like its predecessor, Half Life 2 was met with near-unanimous critical acclaim, and was praised for its advanced physics, animation, sound, AI, rendering, and narrative. The game won 39 "Game of the Year" awards, and several publications have named it "Game of the Decade". Over 6.5 million copies of Half-Life 2 were sold at retail by December 3, 2008, making it a best selling PC game. This figure does not include the significant number of sales through Steam.

Half-Life 2: Episode Two is the second installment in Valve Corporation's series of episodes for the first person shooter video game Half-Life 2. The first two episodes of Half-Life 2 were developed concurrently by separate teams. This episode was released as a part of a bundled package, The Orange Box. A separate copy of Episode Two is available via Steam worldwide.
Continuing with Valve's policy of orienting each episode around a particular theme or set of technologies, Episode Two aims to focus on expansive environments, travel and less linear play. Following the closing events of Episode One, it sees Gordon Freeman and the series' other major players moving away from City 17 to the surrounding countryside.