The Valve Corporation has released its award-winning game Portal as a free game for a limited time until May 24 for both PC and Mac users. As long as the game is downloaded before then, it will be free to play forever.

It was a critical and commercial success upon its inclusion in 2007's Orange Box bundle of Valve games, picking up countless awards for its inventive gameplay and entertaining storyline, and later gaining a standalone release in 2008.

The player starts off running portal experiments on behalf of an advanced computer, using a special gun to connect distinct locations via the creation of portals, but the computer's motivations are not as straightforward as they first appear. To the delight of reviewers and fans alike, the relationship between the player's character and the computer takes an amusing turn for the worse as the experiments become more and more dangerous.

Valve's Steam Network, which started out as an online retailer and social system for PC gamers, was made available for Apple Macs on May 12. Though Macs are not usually thought of as a mainstream gaming platform, Steam's expansion is poised to change that.

The company has gained a reputation for deep discount sales that draw new users to sign up with the Steam Network, though giving away one of its own most treasured titles is an audacious move. Valve is also allowing users who purchase a game via Steam for PC to download a complimentary copy for Mac (and vice versa) where available.

The next Portal game, Portal 2, is on the horizon and further announcements regarding its late 2010 release are expected during the Electronic Entertainment Expo in Los Angeles, June 15-17.

Useful link: Download Portal from the Steam Network store.