Eclipse Mosquitto is an open source (EPL/EDL licensed) message broker that implements the MQTT protocol versions 5.0, 3.1.1 and 3.1. Mosquitto is lightweight and is suitable for use on all devices from low power single board computers to full servers.
HiveMQ is a MQTT broker, tailored specifically for enterprises which find themself in the emerging age of Machine-to-Machine communication (M2M) and the Internet of Things.
VerneMQ is an open source, high-performance, clusterable MQTT broker. It scales horizontally and vertically on commodity hardware to support a high number of concurrent publishers and consumers while maintaining low latency and fault tolerance.