RabbitMQ
RabbitMQ is a software message broker based on the AMQP standard, a replicable message-oriented middleware. Created on the basis of the Open Telecom Platform system, written in Erlang, uses Mnesia as a database engine for storing messages.
Consists of a server, HTTP, XMPP and STOMP protocol support libraries, AMQP client libraries for Java and .NET Framework, and various plugins (such as plugins for monitoring and control via HTTP or a web interface or the "Shovel" plugin for messaging between brokers). There is a client implementation for accessing RabbitMQ for a number of programming languages, including Perl, Python, Ruby, PHP. Horizontal scaling is supported for building cluster solutions.