A music tracker (short version tracker) is a type of music sequencer software for creating music. The music is represented as discrete musical notes positioned in several channels at discrete chronological positions on a vertical timeline. A music tracker’s user interface is usually number based. Notes, parameter changes, effects and other commands are entered with the keyboard into a grid of fixed time slots as codes consisting of letters, numbers and hexadecimal digits.[1] Separate patterns have independent timelines; a complete song consists of a master list of repeated patterns. (Wikipedia)
Here are some of the best (partly free):
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Klystrack;
Multi Synth & Sampler Engine
https://kometbomb.github.io/klystrack/
https://www.youtube.com/watch?v=v2BLJIH9WcA
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SunVox;
Modular Studio
http://www.warmplace.ru/soft/sunvox/
https://www.youtube.com/watch?v=mEdXXKk1hRI
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Hivelytracker;
Amiga SID/ AHX
http://www.hivelytracker.co.uk/
https://www.youtube.com/watch?v=GGwiISMBdwc
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Raster Music Tracker;
Atari Pokey Chip
http://raster.infos.cz/atari/rmt/rmt.htm
https://www.youtube.com/watch?v=MzWPbkLGA08
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Deflemask;
Multi-Chip Engine
http://deflemask.com/
https://www.youtube.com/watch?v=U2tjslu0Xjc
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1tracker;
1bit multi-beeper
https://shiru.untergrund.net/software.shtml
https://www.youtube.com/watch?v=T42WuUpBuHE
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OpenMPT;
Sample/Plugin Module Tracker
https://openmpt.org
https://www.youtube.com/watch?v=utT1NuGPfdM
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(Source: IJiabl0-9)