Valve is ready with an offer that should be hard for anyone with a PC or Mac to resist, offering up the original Portal for the low, low price of free.
The offer stands until the 20th of September, and is part of a promotion for learnwithportals.com - an initiative that uses videogames as part of the education of schoolchildren.
In other words, if you have yet to install Steam now would be the time to do it.