Portal is free on Steam

From now until September 20, you can download Portal for absolutely free for your desktop pc or notebook.

In case you're not familiar with it already, Portal is regarded by many pc gamers to be one of the best games of all time. GameSpot awarded it a 9.0 and an Editors Choice while IGN said it is "quirky, clever, polished, and presented with a spark of a subtly evil humor."

Aggregator	Score
GameRankings	89.15%
Metacritic	90/100
Review scores
Publication	Score
1UP.com	A
Eurogamer	9/10
GameSpot	9.0/10
GameSpy	4.5/5.0
IGN	8.2/10

The game primarily comprises a series of puzzles that must be solved by teleporting the player's character and simple objects using "the handheld portal device", a device that can create inter-spatial portals between two flat planes. The player-character, Chell, is challenged by an artificial intelligence named GLaDOS (Genetic Lifeform and Disk Operating System) to complete each puzzle in the Aperture Science Enrichment Center using the portal gun with the promise of receiving cake when all the puzzles are completed.

The game was originally part of Valve's The Orange Box bundle that also included the now F2P Team Fortress 2, before later becoming a $20 standalone game for the PC and Mac computer system. The sequel of the game, Portal 2 is priced at $29.99.