mqtt-spy is an open source utility intended to help you with monitoring activity on MQTT topics. It's been designed to deal with high volumes of messages, as well as occasional publications.
mqtt-spy is a JavaFX application, so in theory should work on any operating system with an appropriate version of Java 8 installed.
mqtt-spy-daemon is a Java-based command line tool that does not require a GUI environment. Basic functionality works with Java 7, whereas some of the advanced features like scripting require Java 8 to be installed.
For more information on the available functionality see the project's wiki at https://github.com/eclipse/paho.mqtt-spy/wiki.
If you find mqtt-spy helpful in your day to day work, please consider a donation at http://www.justgiving.com/mqtt-spy. All money goes to UNICEF.
For latest mqtt-spy and mqtt-spy-daemon downloads, go to https://github.com/eclipse/paho.mqtt-spy/wiki/Downloads.