Most people would gladly pay money for Portal. However, Valve doesn't want your money. For some reason, they've decided to offer their acclaimed first-person puzzle-platformer for free.

From now until September 20th, you can download the game for PC or Mac for no charge. All you need is a Steam account. That's the only "catch" to speak of. Signing up for Steam pretty easy hoop to jump through, though, considering you'll get a free game out of the arrangement.

In Portal, you play a test subject employed by Aperture Science. You traverse test chambers filled with obstacles by using a Portal gun, which lets you create two-way portals with the push of a trigger.

Portal's sequel, Portal 2, was released earlier this year. If you end up liking the first Portal, I strongly recommend you pick up P2 as well.