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.
