Portal is one of the best games Valve has created. It's almost -- almost -- enough to make people forget that we still haven't got Half-Life 2 Episode 3. Its minimalist storytelling coupled with its brainteasing puzzles made it a memorable experience that has never been replicated -- except by its own sequel.

If you're one of the people who have never experienced the Portal phenomenon for yourself, you now have the opportunity to get involved for precisely no cost whatsoever. That's right, Valve has released the original game for free, and without any fanfare or even a front page feature on the Steam store.

It transpires that the freebie Portal is part of Valve's Learn with Portals campaign, intended to encourage educators to consider the use of video games such as Portal in their classroom. Valve believes that its games have a place as classroom learning aids in the fields of physics, math, logic, spatial reasoning, probability and problem-solving. And they have a point, too -- the Portal series has always encouraged gamers to think laterally and use their knowledge of the game's physics to achieve their goals rather than shooting everything in sight. Not only that, but older, more advanced students can learn a lot about game design, 3D modelling and programming from modifying the game, too.

The promotion, whether or not you're involved in education, only runs until September 20. So if you've not yet faced off against GLaDOS' tests, now's the time to get stuck in. Grab your free copy here.

This article originally appeared on GamePro.com as Portal is Free in the Name of Education Until September 20