Portal has been downloaded 1.5 million times since Valve made it free last week to promote the launch of Steam for the Mac.
According to a post on the Steam website, the game is also only one fifth as likely to crash on Mac as on Windows.
Other cool stats: around two thirds of Steam's new Mac userbase are running it on laptops, and over 11 per cent of sales on Steam in the past week have been for Mac.
Portal is free until 24th May on both PC and Mac.
It's one of the games that uses the "Steam Play" feature, whereby buyers - or in this case downloaders - can access the software on either PC or Mac without having to pay for both separately.