ZeroMQ

ZeroMQ is messaging for people who think RabbitMQ is too much software. It's a socket library that gives you distributed messaging patterns (pub/sub, request/reply, push/pull) without a broker, without a daemon, without configuration files. Just link the library and go.

If you're building distributed systems that need low-latency message passing between components — think microservices talking to each other, worker pipelines, or real-time data feeds — ZeroMQ is the lightweight option. RabbitMQ and Kafka are the heavyweights with persistence, delivery guarantees, and management UIs. NATS is the modern alternative with similar simplicity but better clustering.

The power is the pattern library. Pub/sub, dealer/router, pipeline — these are battle-tested distributed patterns you'd otherwise implement badly yourself. Bindings exist for every language.

The catch: no broker means no persistence and no delivery guarantees by default. If a subscriber is offline, messages are gone. MPL-2.0 license means modifications to ZeroMQ itself must be shared. And the documentation, while extensive, assumes you understand distributed systems concepts.