RabbitMQ is a free and open source message queuing server. It's written in Erlang, which is the same robust language that WhatsApp uses for its messaging backend.
RabbitMQ is currently maintained by Pivotal (whose labs also make the Pivotal Tracker agile project management tool), but it was originally made by LShift. It was then acquired by VMware before being spun out as a joint venture. It's distributed under the Mozilla Public License (MPL) v1.1, an older version of the license that the Firefox web browser uses.
The messaging server can be used from many different languages and frameworks such as Java, Ruby, and .NET. This can make it helpful for linking diverse applications together, for example, a Rails app that you want to ...
