Also check out this Erlang/OTP AMQP server: http://www.rabbitmq.com

AMQP is very interesting. Check it out if you are building a distributed system or if you have a requirement for reliable asynchronous messaging. When I last looked into it about a year ago, the implementations (all two of them that I found - apache qpid and rabbitmq) were still a little rough but it looks like a lot has happened.

It's wierd how few people in the web world are aware of asynchronous messaging.