Eclipse Mosquitto (TM) is an open source (EPL/EDL licensed) message broker that
implements the MQTT protocol versions 3.1 and 3.1.1. MQTT provides a
lightweight method of carrying out messaging using a publish/subscribe model.
This makes it suitable for "Internet of Things" messaging such as with
low power sensors or mobile devices such as phones, embedded computers or
microcontrollers like the Arduino.

Mosquitto is an iot.eclipse.org project.