MQTTX

MQTTX is a client for working with MQTT brokers during development and testing. Each connection gets its own conversation-style pane where sent and received messages appear in order, which makes it easy to keep several clients open at once and check how topics, retained messages and MQTT 5.0 properties behave.

It is made by EMQ, the company behind the EMQX broker, but connects to any standard broker. Payloads can be formatted as JSON, Base64 or hex, and the same project also ships a command-line client and a web version. Mac builds are on GitHub releases, the official site and Homebrew.