I just want to add to Ken's response. ActiveMQ is becoming a popular JMS container in the open source messaging framework category. It's easy to unit test with, because of its support for embedded broker. It also has fine grained security control at different levels of JMS architecture (i.e. Broker, Connection and Message level). If you are interested in learning more about ActiveMQ, checkout their website or ActiveMQ in Action book from the same publishers as our book (Manning).