Sun's Open source OpenMQ (https://mq.dev.java.net/). You can get free and paid support for the same.
See this blog post about some comparison with ActiveMQ, etc -- http://alexismp.wordpress.com/2008/06/06/openmq-the-untold-story/.
I've heard that OpenMQ is more stable.
ActiveMQ is more flexible. as in, you can use it with more languages. There are probably more people on ActiveMQ's mailing list than OpenMQ.