Thought I'd post this since it's such a great game and it's been mentioned on the show a fair bit.
Steam has Portal for half price at $9.99 for this weekend. (PC version, obviously)
It's short, sweet and unmissable and there's plenty of high quality user made maps available for when you finish the main game.
This isn't spam, I'm just doing my geek duty.
TRAILER!
Oh and while I was there, I saw
this. Abe's Odyssey
and Exodus for $14.99 minus 10%! Oh Valve, I wub woo so much.