I want to add my own implementation to that list:
https://github.com/prof7bit/fpc-mqtt-clientIt is very new (3 days at the time of this writing) and currently implements
only user/pass authentication,
no ssl (yet) and
no QoS (yet).
It does not need any additional networking library and needs nothing installed in the IDE because it uses only TInetSocket from the SSockets unit. It implements protocol version 5. The license is LGPL v2 with static linking exception.
It can already connect, subscribe, publish, it supports the MQTT5 features "topic alias", "response-topic" and "correlation-data" and I am currently testing on Linux against mosquitto 2.0.x (the one that comes with the home assistant addon).
Edit: 17.11.22: SSL implemented
Edit: 18.11.22: client certificate authentication implemented
Edit: 22.11.22: QoS 1 and 2 implemented