I’m pretty sure most people here knows and loves MQTT Explorer.
AFAIK, it is simply the best tool for MQTT debugging, monitoring, topic exploring etc.
The original developer, Thomas Nordquist, is now seeking for help in maintaining it:
https://github.com/thomasnordquist/MQTT-Explorer/issues/885
I am no programmer, so the least I can do is help spread the word about it.
If someone here like me loves MQTT Explorer and is willing to help, in any way, let me or Thomas know.
Thanks! 
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It would help if he opened the license. It’s a great project but suffer from a licensing that prevent public collaborations