If you're after Narbacular Drop, the game which is to become Portal, give the creators' (who Valve hired to make Portal) site a rest by grabbing the game from one of the mirrors:

(VGPro Mirror)
(PlanetMirror Mirror)
(FilePlanet Mirror)
(DigiPen Mirror)

The original site will be online at http://www.nuclearmonkeysoftware.com/narbaculardrop.html once per day between ~0200 GMT and whenever their usage hits 37GB, but they're not hosting the game on that space any more, so that people can reach the mirrors for longer.

For those who don't know, Portal is a new first-person game coming to steam to be distributed with Half-Life: Episode Two, in which the player navigates obstacles by means of a gun which places portal entrances and exits.