ProtrackerSTE is an upgraded port of ProTracker from the Amiga. It handles only 4 tracks, but has the full PT command set and gives good quality playback. (Note there are also a bundle of PT clones for these machines, such as Esion)

Octalyser is an 8-track tracker. You need a fast STE or a TT to playback all 8 tracks though. Octalyser will let you create or play back 4, 6 or 8 track mods. It will load many different mod formats, but not S3M or MTM. Sound quality is quite good.

Octalyser (mentioned above) has drivers for the enhanced sound system, thus giving good quality playback, but only 4, 6 and 8 track mod formats, not S3M or MTM.

MegaTracker - 32 track tracker. All tracker commands and practically all mod formats are supported including S3M and MTM. Unbelievable speed and quality are produced by the DSP mixing. Unfortunately, the instructions have not yet been translated from french, but it is only a matter of time.

Digital Tracker - 32 track tracker. Supports almost all tracker commands and formats, although it hangs on some S3Ms. this is commercial software and only a demo is available, which has certain restrictions.

DBE Tracker It is worth noting that this is the newest multichannel tracker for the ST. Unlike Mega and Graumf Trackers, DBE will run on ALL Atari machines and will attempt to play up to 32 channels on any of them. Obviously there is a serious loss of quality on lower spec machines... A demo of the programme is available from many Atari sources and it can be registered with the author (Denis Huget) for about 6 UK pounds. Updates are provided on a regular basis to registered users.

http://sectorone.atari.org

One mod format not supported by any Atari trackers/players is .IT - I have asked Denis to add it to DBE !!