I've been developing a MQTT GUI client called MQTTy (the name is not very original but I needed the word MQTT in it). That will help me and hopefully help others, to develop, test and debug their IoT applications that utilizes the MQTT protocol.
It's written in Rust, completely free software, licensed under the terms of GPL 3.0 or later versions.
Currently the app does not hit the MVP stage, so there is a lot of work to do to reach it.