The music from the Beyond Good & Evil soundtrack, written and performed by Christophe Heral, is incredible. It has haunting ballades, adrenaline-filled anthems, and even some surprisingly good French hip-hop. It was praised unanimously in reviews as one of the best parts of the game.
Unfortunately, you can't buy it anywhere. Due to a flood of fans clamoring for it, though, you can download it. Ubisoft released it on Nintendo Insider as a collection of 31 freely downloadable mp3s. 256kbps bitrate, no less! Very classy move on Ubisoft's part.
Also, if you're a purist, and you own the PC version of the game, you can use Rahly's bfextractor tool to extract the individual music tracks directly from the CD.
See also:
wget --recursive --level=1 http://videogamesmedia.com/gcn/beyondgoodandevil/