eMQTT5 is a MQTT cross platform client though for embedded system with minimal ressource usage and complete coverage of the specification. If you are searching for a MQTT client that's working with latest standard, with all features and without blowing your microcontroller's flash and memory budget, look no more.
MQTT is a protocol targetted for internet of things (IoT). It allows small CPU to exchange messages efficiently over unreliable connection. It's a de-facto standard for home automation.
The source code is here.
A good documentation is here.