http://kgs.kiseido.com has a great tutorial for it (link on the left). The rules are really simple, but when you play the game... wow! It's sort of like Chess in that you have to plan and think ahead, but I feel like I have to think much, much further ahead than I ever did in Chess.
So if you want to have some fun, stretch your brain a bit, and learn an ancient game, go check Go out!
(I feel like I'm writing a high school essay or something...)
ETA: Overheard coversation, two guys talking across the hall from me. They're pointing to a whiteboard, but I can't see what's on it.
Man 1: So this guy here, he's to big for any hole...
Man 2: Uh huh. And this one?
Man 1: He fits, so let's move him in and put him in the first guy's position.
Man 2: Then we'll need to find a bigger hole for that first guy...
Snicker. It's about circuits or boards or something, but still! Hee.