MQTT is a publish-subscribe protocol that's perfect as a broker for device to device communication. Created by Andy Stanford-Clark and Arlen Nipper, it has been used in projects like Andy's own 'Twittering House'. You can see more from Andy in this TEDx talk:
Mosquitto is an open source MQTT broker implementation that's under active development, has an established reputation, and is well documented. I've done some informal testing with Mosquitto and it seems like the perfect broker to use in this project.