"Portal" was the first Valve-developed game to be added to the OS X-compatible list of games available on the launch of the Steam client for Mac on May 12, 2010, supporting Steam Play, in which players that had bought the game either on a Macintosh or Windows computer could also play it on the alternate system.As part of the promotion, "Portal" was offered as a free title for any Steam user during the two weeks following the Mac client's launch.