Wikipedia
Protracker
ProTracker is a popular freeware tracker created by Lars Hamre, Anders Hamre, Sven Vahsen and Rune Johnsrud for the Amiga platform. It is among the first programs that allowed for widespread creation of music without studio equipment. It was popular for amateurs and professionals, and set a standard for the MOD fileformat.
There's an ongoing effort to make a clone of ProTracker for modern platforms at SourceForge.