RabbitMQ

With RabbitMQ your applications gets a common platform to send and receive messages, and your messages a safe place to live until received. Allowing your application to handle long-running or blocking tasks, and respond quickly to requests instead of being forced to perform computationally intensive tasks on the spot. The software is developed and released as open-source and is currently the most widley used free and open-source message-broker. Commercial, enterprise releases of RabbitMQ is also avalible for a subscription fee.