Mosquitto is an open source (BSD licensed) message broker that implements
the MQ Telemetry Transport 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 "machine to machine" messaging such as
with low power sensors or mobile devices such as phones, embedded
computers or microcontrollers like the Arduino.

WWW: http://mosquitto.org/