Rock Paper Shotgun has spotted that Portal is free on Steam. It's a limited offer, set to expire on September 20, and is part of Valve's educational initiative, Learn With Portals, which aims drive student interest in STEM subjects - science, technology, engineering and maths - by getting them to create Portal levels.
Portal free on Steam
Free until September 20 as part of Valve's Learn With Portals initiative.
Source: Rock Paper Shotgun