In terms of specific protocols, e.g., transport related, you should check out our messaging related projects and technologies such as HornetQ, ActiveMQ/A-MQ, QPid/MRG-M and of course the OASIS AMQP protocol.
The Message Queuing Telemetry Transport (MQTT) protocol is an OASIS standard and one of the more popular M2M protocols. It is supported in ActiveMQ/A-MQ and Dhiraj has written about our efforts in the interoperability Test Day at EclipseCon 2014. There's an official press release, a report about the event and a blog by the organiser.