Valve has announced that Portal has been downloaded 1.5 million times since it became free last week. The title is being given away until 24th May to promote the launch of Steam for Mac. Apparently, the game is one fifth as likely to crash on Mac as Windows. Around two thirds of Macs on are running Steam on laptops and over 11% of sales on Steam over the past week have come from Macs. Portal’s Mac release was designed to show off the Steam Play feature, where gamers can access the game from both PC and Macs without having to pay for both individually.