An extended PC demo of Valve Software's 2007 hit (and Shacknews Game of the Year by both readers and editors) Portal (PC, PS3, X360) will be made available for free to gamers with NVIDIA graphics cards, the developer announced today.

The demo, called Portal: First Slice, comes as part of a recent agreement between Valve and NVIDIA to "collaborate on development, distribution, events, and more." Gamers equipped with NVIDIA graphics cards looking to give the test chambers and deathtraps of the Aperture Science labs a trial run must allow Steam to auto-detect their NVIDIA cards to unlock the demo.

First Slice and the collaborative campaign between the two companies marks a departure from the former marketing team-up between ATI and Valve, shifted by an increase in NVIDIA hardware use among Steam gamers. "More Steam gamers, including myself, play on NVIDIA hardware than any other GPU," said Valve marketing VP Doug Lombardi, citing Valve's frequent gamer hardware surveys.

Portal: First Slice is available for immediate download from Steam—provided, of course, that you've got an NVIDIA card under the hood. Like Valve's previous free game offer for ATI users, this download also includes Half-Life 2: The Lost Coast and Half-Life 2 Deathmatch, as well as Peggle Extreme.