Valve have made "volume 1" of the soundtrack to Portal 2 available as a free official download.  It's 22 tracks, 74 minutes of 320kbps MP3s in a 170MB zip file.
http://www.thinkwithportals.com/music.php

I've yet to play the game but I expect to be familiar with the music shortly.

Excellent! Downloading now....
...
Deleting now...
Well, this isn't the type of stuff I enjoy listening to outside of a game. "The Courtesy Call" caught my interest, at least.

Awesome! Downloading right now.

I'm not usually a fan of any type of ambient music, but once in a while it wins me over: Silent Hill, Shadow Hearts, Half Life 2, etc. The first Portal also fell into that category and the second one went a step further and really, truly impressed me. I didn't just enjoy the music, I loved it.

Can't wait to start listening!