Portal
Download it in High Definition with Steam, or visit its web page to stream it.
On July 13th, 2006 at EA's Summer press event, Gabe Newell announced that a game tentatively titled "Portal" was in production, and that it was due for release alongside Half-Life 2: Episode Two and Team Fortress 2 before the end of the year.
The game is described as a first-person shooter set in the Half-Life universe where you use an "Aperture Science Handheld Portal Device to create dimensional doorways." [1] The game operates as a puzzle-based shooter, allowing you to create portals that link to one another on any flat and large enough surface.
The gameplay is similar to Narbacular Drop by Nuclear Monkey Software, an indie game that won the IGF 2006 Student Showcase. Members of the development team that created the game were hired by Valve in 2005.