Phill Niblock is known for his use of looooooooooooooong tones. I've heard a couple of his pieces that are just an instrument playing a single note for 30+ minutes until it detunes itself. Not interesting, really. This guitar-drone piece kinda plays off the same theme, but I really like it. The disc contains two different interpretations of the same 30-minute work. One by Rafael Toral and one "massed" version featuring Alan Licht, Kevin Drumm, Lee Ranaldo, and Thurston Moore that evolves slowly, almost imperceptibly, but above all satisfyingly. It took me forever to dig this one up, so I'll spare you the trouble.
http://www.mediafire.com/?kaynoqymjmg
http://www.mediafire.com/?kaynoqymjmg