This was two years ago. JBossMQ was being rewritten and the new version was too immature.
Ah, ok... gotcha. Didn't realize the time-frame.
I don't remember HornetQ.
It's fairly new... basically the successor to JBossMQ. It is reputed to be blazing fast though. I've been experimenting with it, but haven't used it in anger.
We investigated RabbitMQ, which seemed good but still a bit young to bet our company on.
Fair enough.
I just found a thread I started about ActiveMQ and scaling:
http://activemq.2283324.n4.nabble.com/6000-ActiveMQ-clients-...
Note the last message there. 36,000 threads when using NIO! And these answers were from the people commercially supporting ActiveMQ (FuseSource). I never did find out (on that thread or elsewhere) whether ActiveMQ has ever been used in a situation that large. I suspect it hasn't.