"Fratres" by Arvo Pärt
Drony, haunting medieval-modern classic played with two SID chips. About 10 minutes--very nicely done. There is a version for a .sid file player but I just downloaded the .mp3. The arrangement is by Linus Åkesson. The organ sounds produced by the chip make the piece reminiscent of early Terry Riley.
Thanks to drx from Bodenstandig 2000 for mentioning this.