"A lovely bit of news from RPS this morning. If you don’t already own it, you can download Portal for nothing on Steam. It’s being made free to download until September 20. If you download it before then, you’ll own it forever.
It’s all part of Valve’s Learn With Portals initiative, which aims to promote Portal’s reality bending puzzles as an educational tool, and hopes to encourage the next generation to start building a new wave of even more dastardly test chambers."
For more info on Portal in education, see http://www.learnwithportals.com/