Good price, and better than the first Portal game IMHO. http://store.steampowered.com/app/620/?snr=1_620_4__46
Already got it, still havn't played it! Its on my list of "Games to play" I would love to play it cooperative. By the way, its a HUGE download, something like 12 Gigs.
I have played that game quite a lot, actually. Just haven't tested it on Linux yet since I know valve will release a Linux version some time. It is a very entertaining game, and the cooperative mode is just plain awesome.