Portal free on Steam

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.

http://www.youtube.com/watch?v=QGQo0z3XikI