Really Small Message Broker (RSMB) is an MQTT/MQTT-SN message broker which was originally developed by IBM and made freely (without cost, but not open source) available for personal use/evaluation. Its source code were at some point contributed to Eclipse (under EPL/EDL license) as a Mosquitto subproject (see Eclipse proposal). However, its development has stalled and the original author Ian Craggs now recommends using MQTTSNGateway for MQTT-SN support (which is its most prominent feature compared to Mosquitto).

Watch a demonstration video of RSMB in action.