logic4mqtt - Logic and scripting engine for use with MQTT. Uses Java's general scripting interface, so scripts can be written in a multitude of languages like Javascript, Groovy etc.
mqtt-scripts - Logic and scripting engine for use with MQTT. Node.js based, require command works as expected.
influx4mqtt - Insert incoming MQTT values into InfluxDB.
Misc
kodi2mqtt - Interface between a Kodi mediacenter instance and MQTT.
homekit2mqtt - Interface between HAP-NodeJS and MQTT. Can be used to control MQTT attached devices with Siri
airtunes2mqtt - MQTT controlled Multi-Room Audio with Airplay/Airtunes Devices.
List of Software (maybe) otherwise usable
Node-RED - A visual tool for wiring the Internet of Things
mqtt2graphite - Subscribe to MQTT topics and push to Graphite's Carbon server