I would be interested to see a detailed comparison of the above and SQS which I've used to great success with the python library boto (http://code.google.com/p/boto/). And yes, I googled and haven't found anything good :)
I haven't used ActiveMQ before due to no reason other than horror stories from basically everyone I know who has used it heavily.
RabbitMQ seems like an interesting solution to explore if I ever need more speed than SQS provides.
Yea, boto is a great library. IME, SQS seems to be a good option only when you have low throughput. I tried running a high volume of jobs through SQS and found that I needed a lot more workers (EC2 instances) to do the work because each message was so slow to pickup. That made things cost prohibitive. I'm currently working on another AWS based project. It was initially using ActiveRecord based db queues but that, of course, failed at high volume. So I replaced it with kestrel, which works with any memcached client, and that's been great. However, kestrel doesn't do topic/group subscriptions where lots of receivers can get the same message. That's where these message servers seem to be potentially valuable options.
I haven't used ActiveMQ before due to no reason other than horror stories from basically everyone I know who has used it heavily.
RabbitMQ seems like an interesting solution to explore if I ever need more speed than SQS provides.