In several of my infrequent discussions of gaming, I've attacked the notion that many people have that games can't be art. There are lots of counter-examples of arresting stories, epic in scope, mounted within a framework of innovative game design.
And then there's Portal. I'm not going to spoil it. What I am going to say is that although it's three hours long, it's still absolutely brilliant. Amazing. Astonishing. Fun, even. It's like the gaming equivalent of a really, really tightly crafted short story with an absolutely kickass ending that raises more questions than it answers.
(There's a longer, spoiler filled discussion of Portal-as-short-story over at Gamers with Jobs. Go read it, but only if you've already finished it, or don't care about spoilers. And you REALLY should.)
It's also hysterically funny, and the ending features the catchiest song you've ever heard. No, really. I defy you to find a more pernicious earworm than Jonathan Coulton's "Still Alive". Even the Weebl crew can't come close.
Somebody should really send this thing to Ebert.