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. A good example of this is all of the work
that Andy Stanford-Clark (one of the originators of MQTT) has done in home
monitoring and automation with his twittering house and twittering ferry.