If you want to learn about Message Queues firsthand (and you should if you care about CS) I recommend implementing some with ZeroMQ. (I love the joke in the article about it being "zero effort" xD) There are many, many forms of queues, and you can implement most of them with ZeroMQ. There's tons of examples in many different languages, and lots of guides and docs to help you. Even considering all that, it will still be a pain to get the queues working, and that pain will help reinforce the purpose behind different queue architectures, and why it's so nice to have COTS message queues.
That said, if you want a customizeable FOSS message queue stack, use NATS. I'm not aware of a better, more complete solution designed for custom integration.
I never used ZeroMQ (visited their webpage many many times though) but I totally agree that going through the pain, struggles and subtleties of message queuing is a very valuable and important learning experience.
I had those experiences with RabbitMQ around 10 years ago when I pushed the team I worked for towards a message queue solution to scale our distributed test system. It turned out successful but it took quite a while :)
That said, if you want a customizeable FOSS message queue stack, use NATS. I'm not aware of a better, more complete solution designed for custom integration.