HornetQ is an open source project to build a multi-protocol, embeddable, very high performance, clustered, asynchronous messaging system.
HornetQ is designed with usability in mind.
HornetQ is an example of Message Oriented Middleware (MoM) For a description of MoMs and other messaging concepts please see the chapter 2 of the user manual "Messaging Concepts".
HornetQ takes over from JBoss Messaging as our premier JMS implementation.
HornetQ (NOT HotnetQ!) is a quite a decent messaging provider. As the default JMS provider in JBoss AS it does its job well, but it seems the developers are more concerned about their standalone "core protocol" than they are about the JMS API and JBoss AS integration. There is a hideous bug in their so-called JMS bridge implementation for which a patch has been submitted ages ago, but "they" just won't apply it, which is a shame.
Other than that it's fast and works well. Have been using this in production for some time now and except for the mentioned JMS bridge bug did not encounter much problems.