ZeroMQ

ZeroMQ is a messaging library that acts like sockets on steroids, passing messages between processes reliably without a broker. No server to install, no broker to maintain. Just embed it in your app and start sending.

ZeroMQ gives you patterns: pub/sub, request/reply, push/pull, and more. It handles reconnection, message queuing, and load balancing at the library level. Works across TCP, IPC, in-process, and multicast. Bindings exist for basically every language.

Everything is free. MPL-2.0 license. No hosted version, no paid tier. It's a library you link into your code. Solo developers building distributed systems should seriously consider this before reaching for RabbitMQ or Kafka. Small teams benefit from the zero-ops angle: there's no broker to babysit. At scale, though, you'll eventually want the durability and management features that a real broker provides.

The catch: no message persistence. If a consumer is down, messages can be lost. And "brokerless" means YOU are responsible for topology and failure handling.