ZeroMQ — An open-source universal messaging library

ZeroMQ (also ØMQ, 0MQ, or zmq) is an open-source messaging library that describes itself as looking "like an embeddable networking library but act[ing] like a concurrency framework." It gives you sockets that carry atomic messages across in-process, inter-process, TCP, UDP, TIPC, multicast, and WebSocket transports, connectable N-to-N with patterns such as fan-out, pub-sub, task distribution, and request-reply.

The pitch is a tiny asynchronous-I/O engine that scales to multicore applications built as asynchronous message-processing tasks, with language APIs for a wide range of languages and support for most operating systems. The homepage leads with the message patterns (pub-sub, push-pull, client-server), the breadth of transports, and an active open-source community. The canonical guide promises "60+ diagrams and 750 examples in 28 languages."