MIMIC MQTT Simulator – Thousands of publishers and/or subscribers for rapid prototyping of IoT Applications, performance testing and tuning of deployments.
Bevywise IoT Simulator – IoT Simulator provides complete functional and performance testing tools for the MQTT Platform, Application & Devices Development.
IA92 – IA92 support pack includes very useful MQTT Java swing GUI for publishing & subscribing.
MQTT.app – an OS X desktop test application (simple pub/sub) by 2lemetry, available from the Mac App Store.
SimpleMessage – a basic OS X desktop client based on the Paho C client.
TT3 – a full featured windows MQTT client application using Paho libs. Several additional features like performance testing and alerts.
mqtt-spy – the most advanced open source utility for monitoring activity on MQTT topics; based on the Paho Java client; for details see the project’s home page.
Xenqtt – includes a client library, mock broker for unit/integration testing, and applications to support enterprise needs like using a cluster of servers as a single client, an HTTP gateway, etc.
twitter2mqtt – a Twitter to MQTT gateway (1-shot) which is using mosquitto.
mqtt-http-bridge – this simple web application provides a bridge between HTTP and MQTT using a RESTish interface.