just a quick notice on the newest version of the HiveMQ MQTT broker , because I think the features introduced in version 1.4 could be of interest to people on this list.
Since version 1.4 an open plugin SDK is available on GitHub , which enables everybody to write own plugins. With plugins it is possible to implement own authentication, authorization logic and to execute custom business logic on certain events (broker start/stop, client connect, client disconnect, publish message, subscribe message , ...). We also open sourced a sample plugin  for reference and a Maven plugin  for a better development experience. More plugins  will be following in the next few weeks.
Since 1.4 HiveMQ is supporting native websockets, which means every web browser can communicate with the broker over MQTT without an extra webserver. Our public broker at mqttdashboard.com
already supports this. If somebody want to try it, server is broker.mqttdashboard.com
and port for websockets is 8000.
More insights can be found in the release blog post  and the documentation .
HiveMQ is free for personal use and testing until 25 simultaneously connected clients. If you want to use it in a cool open source project, please contact me, I'm sure we can work out a suitable solution.
Also if you have any questions, suggestion or other feedback, please don't hesitate to drop me a line.