Download of MQTT Spy 1.3 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. This free software for Mac OS X was originally developed by mightyIT. We recommend checking the downloaded files with any free antivirus. The software is included in Developer Tools.
From the developer:
MQTT Spy is used for testing pub/sub protocol with MQTT. All you need to do is create a MQTT connection with broker and test MQTT messaging. With this application, you can control any MQTT enabled IoT devices and monitor sensors with pub/sub protocol.
You may want to check out more Mac applications, such as MQTT, Spy vs Spy or Plug Spy, which might be similar to MQTT Spy.