Portal

Portal is the original game of the Portal series. It is a single-player puzzle-based video game developed by Valve and released as part of The Orange Box on October 9, 2007 for Microsoft Windows and Xbox 360; the PlayStation 3 version released at the later date of December 11, 2007. Portal was also re-released as part of the launch of Mac OS X support for Steam on May 12, 2010; to celebrate it was given away for free until May 24, 2010 to Steam users on a promotional page on the Steam website. Further to this, Portal has been made available for free again for a limited duration as part of the Learn With Portals project in 2011.