They solve slightly different (architecture) problems and they are on completely different ends of the message queue library spectrum of "usability vs. customiz

They solve slightly different (architecture) problems and they are on completely different ends of the message queue library spectrum of "usability vs. customizability". RabbitMQ is a "batteries included" solution. ZeroMQ is a roll your own sort of library. If you just want a message queue use RabbitMQ. If you want to build your own message queue system (with complex or specific requirements) use ZeroMQ.