Here you can add links to defMON material available elsewhere on the net.
Here are some links to music made with defMON:
The sid chip is the sound chip of the Commodore 64. It exists in two main revisions, called 6581 and 8580, and there are some sub-revisions as well.
JCH wrote a nice comparison of the features of different C64 music editors, including defMON: