Eclipse Mosquitto is a project which provides an open source MQTT broker, a C and C++ library for MQTT client implementations and the popular command line MQTT clients.

Eclipse Paho is an umbrella project on a mission to provide high quality implementations of tools and libraries for M2M communications.

HiveMQ is an MQTT broker and a client based messaging platform which uses MQTT protocol for fast, reliable and efficient bi-directional data transfer to and from IoT devices.

MQTTBox is a cross platform application that makes it simple to create MQTT clients, virtual device networks, and load test MQTT devices and brokers.