QMON, along with its PE companion JMON, is probably the most advanced debugger for the QL. QMON uses the existing windows within S(uper)Basic while JMON comes with its own PE windows.
For the first time these tools are now freely available and thanks to Oliver Fink there is now also a newly edited version of the original manual available.
qmon211_jmon214.zip (as last distributed by Jochen Merz)
qmon213_jmon216.zip (assembled by me from source)
In the last 15 years I have made some additions to the tools, but these are not yet ready for a general release, watch this space…