To celebrate the launch of its digital distribution platform Steam on Mac computers, Valve has made downloads for its award-winning first-person action game Portal free to both Windows and Mac users for two weeks. Valve made the platform available to Mac users earlier today with an initial selection of 57 games (compared to the 1,100+ releases available to Windows users), including Runic Games's Torchlight, Amanita Design's Machinarium, Firaxis's Civilization IV: The Complete Edition, and several classic LucasArts adventure games. All 57 titles take advantage of a "Steam Play" feature, which ties games to a user account instead of individual hardware so users can play a game they've purchased via Steam on any computer once they've logged in. With this feature extended to Macs, gamers can now play a title they purchased a Windows version for on a Mac (if a Mac edition is available), and vice versa. Valve intends to release more Mac-compatible versions of other catalog titles and to eventually adopt a simultaneous multiplatform development approach for its games, meaning that it will release day-one Mac versions of its new games starting with Portal 2 later this year. The company has also made the first Portal game available to download for free to all Steam users until May 24th. Initially released to Windows users in 2007 -- with PS3 and Xbox 360 ports, as well as a Still Alive expansion following afterward -- Portal made its public debut on the Mac operating system today with Steam.