ZeroMQ is really good. I think the best introduction to it is this video, http://vimeo.com/20605470 (ZeroMQ is the answer). Just ignore the it was recorded at a PHP conference, the examples are pretty language agnostic.

I think one of the best things about ZeroMQ is there are not external brokers or services that you have to keep running. The entire architecture is captured in your own long running processes.