JAL Software's C64 Public Domain Disk 1 by James A. Lisowski is a nice collection of utilities and BASIC code samples for the Commodore 64. Many of the utilities have been posted here individually, but there are still a few treasures on this disk that you'll want to check out. In particular there is a program called Alternate Scales and another called Alternate Chords that show off the amazing capabilities of the SID sound chip.