If you've never played the original Portal before, then Valve is giving you a chance to rectify this by making it free for a limited time on its digital distribution platform, Steam.
The game will be free to download by anyone with a Steam account until the 20th of September, 2011. After that, it will return to its original retail price.
Valve has decided to launch the initiate in order to help promote its "Learn with Portals" campaign. They hope the game will play a vital role in teaching children about science and engineering.
Source: Learn With Portals