NATS

It handles pub/sub (publish-subscribe, one service broadcasts, many listen), request-reply, and persistent streaming (JetStream) in a single lightweight binary.

Fully free under Apache 2.0. No paid features, no enterprise tier, no cloud lock-in. Synadia (the company behind NATS) sells a managed cloud service and support, but the server itself is complete. Self-hosting is remarkably easy. Download a single binary (11MB), run it. No JVM, no dependencies, no configuration file required for basic use. Clustering for high availability is straightforward. This is one of the easiest infrastructure components to operate.

Solo developers: probably overkill unless you're building event-driven architecture. Small teams: NATS replaces Redis pub/sub, RabbitMQ, and basic Kafka use cases with dramatically less ops. Growing teams: JetStream gives you persistent messaging without running a separate Kafka cluster.

The catch: NATS's simplicity is also its limitation. It doesn't have Kafka's ecosystem of connectors, stream processing frameworks, or exactly-once semantics at massive scale. If you need to process millions of events/second with complex transformations, Kafka exists for a reason.