I think they’re slightly different solutions — ZeroMQ works without a broker, RabbitMQ requires a server process.

If you use the brokerless model, there was a bit of drama over ZeroMQ — the original technical developer (Martin Sustrik) left and created a successor, nanomsg, with what he learned. At some point, Martin lost interest, and Garrett D’Amore took over maintenance and did a rewrite called nng. Both the old nanomsg and nng are maintained, with nng being somewhat actively developed, but also fairly “complete”, so there’s not a lot of excitement like you see with some projects. ;) nanomsg and nng are essentially wire-compatible, so you can mix and match depending on bindings availability for your language.