LavinMQ
Ultra quick message queue and streaming server
...It implements AMQP 0-9-1 and MQTT 3.1.1, making it useful for applications that need reliable messaging, IoT communication, event processing, and backend service coordination. The server is written in Crystal, which helps it keep memory usage low while supporting high throughput. It is designed to handle very long queues, many client connections, and production workloads with minimal configuration. LavinMQ can be used as a lightweight alternative in RabbitMQ-compatible environments where AMQP-based clients are already common. Its main value is combining speed, low resource consumption, protocol compatibility, and operational simplicity in one open-source broker.