RabbitMQ is an implementation of AMQP, the emerging standard for high performance enterprise messaging. The RabbitMQ server is a robust and scalable implementation of an AMQP broker.
Good overall, but they've added 'security' in with the package and it means you can't easiliy use it as a messaging layer inhouse without needing a 'deployment' on remote machines. If you're like me and find this anoying, use ZeroMQ instead.