"Diablo 2" came out 10 years ago and it still costs $30. That's a short way of saying that really good games keep their price tags for a good long while, while other games drop dramatically in price after a few months. "Portal" is one of the best PC games ever devised and Valve could very easily sell copies of it for $10 for, well, ever. But that's not what they're doing, at least for the next few days. Yes, until September 20, "Portal" is completely free on Mac and PC. Wowza.
It seems unlikely that you, someone who is reading a gaming blog, would not already own a copy of "Portal," but maybe you bought The Orange Box on console and wouldn't mind having "Portal" on your home computer or laptop. Say, for example, you recently bought a MacBook Air. Yes, it will run the original "Portal" like an absolute champ. Hell, it'll run "Portal 2" swimmingly, as well.
The promotion is tied in to a new initiative called "Learn With Portals," which encourages schools to teach physics, math and engineering to students using the mechanics in "Portal." Uh, why did I have to play "Oregon Trail" when I was growing up when kids today get "Portal"? O! Cruel fate.
For that small group who has never played the original "Portal," please enjoy yourself and stick with it. It's a delightful concept that doesn't become utterly brilliant until the last act of the game. Thankfully the game is only about 3 hours long, so it isn't much of a time sink. It also provides a great lead-in to "Portal 2," which evolves just about every concept dramatically.