ProTracker was a very a popular freeware tracker for the Amiga platform. ProTracker allows the user to create sequences of notes called “patterns”, which are chained together to form a complete song. It is amongst the programs that first allowed for widespread creation of music without studio equipment (very popular in 1990′s). Music created in ProTrackeruses the MOD file format. The use of original MOD format is somewhat faded over the years, but there are newer software that continue the same idea (for example Jeskola Buzz or SoundBox).
If you want to check other similar software, take a look ar ModplayerJS.