An article by Thomas Beyer (also available in German language) gives a short introduction into popular messaging solutions which are available as free open source projects.
“Newer architectures and the standardized AMQP protocol have led to a flood of message brokers. All brokers take claim to be fast, robust and reliable. But what really distinguish the broker? How do I choose the right broker? Should we continue to use established brokers such as the ActiveMQ or try a more modern one? This article attempts to answer these questions and help the reader in selecting a suitable broker.”