HornetQ

Try out this messaging engine: it has a AIO enabled journal (only on linux using JNI)! This project is the continuation of jboss messaging 2.
I feel this is the killer open source JMS server

Simplicity, usability and performance. It has what it takes to be implemented in complex enterprise enviroments (this is what most CIOs love to hear) and has the flexiblity of programming (this is what developers want) and administration using JMX so you can use standard tools (this is what the ops people need).
Overall the best, but our opinion is biased because we offer services on HornetQ ;)