On July 13th, 2006 at EA's Summer press event, Gabe Newell announced that a game tentatively titled "Portals" 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."  The game operates as a puzzle-based shooter, allowing you to create portals that link to one another on any flat surface large enough.
The gameplay is similar to that of Narbacular Drop, an indie game that won the IGF 2006 Student Showcase. Members of the development team that created the game were later hired by Valve.