The IA92 supportpac includes a light-weight Java GUI for using MQTT.
There is general agreement we should have a replacement utility as a part of paho. IN fact, there is no reason we couldn't have a utility in each of the languages we provide client APIs for.
Before we each go off and write our own gui in our favourite language, we should but some work in up front to design, in a language agnostic way, what the utility should look like and what features it should provide.
As IA92 already exists, we should treat that as the initial straw-man for discussion.
Existing Eclipse Paho Utility
mqtt-spy is an open source utility intended to help you with monitoring activity on MQTT topics. It is based on Eclipse Paho and available at http://kamilfb.github.io/mqtt-spy/.
MQTT.fx is a JavaFX based Paho Utility http://www.jensd.de
JRE bundled installation packages available at http://www.jensd.de/apps/mqttfx/