While it's highly unlikely that anyone here who might care doesn't already know...
Portal (the game) is currently available for
free from Steam (but only if you get onto this and get it before the 24 May). I think pretty much everyone and their grandma has played this (and so they should), but if you're one of the unlucky few who hasn't (like I was), get it now. Link is at:
http://store.steampowered.com/freeportal/Why should you play it? It's an excellent game, with great writing, a clever puzzle gimmick, and great environmental storytelling. It's also relatively short (took me about 3 1/2 hours to finish the main game) so even if you're busy you should be able to find the time. Heck, even Yahtzee (from Zero Punctuation) loves it, and his schtick is that he hates everything, so that's saying something.
I really enjoyed it and as it's fully free and legal to everyone I feel I need to get the word out.