Valve and NVIDIA have announced that all NVIDIA gamers may receive a free copy of Portal: First Slice, a special version of the new game from Valve, by visiting this website and "Taking the test." Steam will then auto-detect the presence of NVIDIA hardware and make Portal: First Slice available immediately, free of charge. To make it easy for all NVIDIA customers to redeem this offer, a link to the Steam offer will also be included in all NVIDIA drivers.
Source
News: Portal: First Slice Free to GeForce Users - GamersHell.com
You can download Portal: First Slice from here
Portal: First Slice is free for all NVIDIA Gamers
Only Nvidia users can download it for free