Getting started with MQTT
MQTT(Message Queue Telemetry Transport) is a Publish-Subscribe based “lightweight” messaging protocol for use on top of the TCP/IP stack.
It is quite useful for connections with remote locations where a small code footprint is required and/or network bandwidth is at a premium.
There are many different Brokers and Clients that implement the MQTT protocol.
A list of Brokers, Clients and Tools can be found on the mqtt.org website here, while it is not definitive it does offer a representative sample. There is also a curated list on github.com on MQTT.
News on MQTT specifications can be found at mqtt.org.
MQTT v3.1.1 is an Oasis standard available here