NATS is an open-source message broker with high performance, clear documentation and very easy to use. The server is written in Go, and client libraries are available in many languages - some of them official, others contributed by community. The Tcl client is available from Github .
Whenever you have a job of building a distributed application or interfacing Tcl with software written in other languages, NATS might be a superior solution to raw TCP sockets, also much simpler than ZeroMQ or RabbitMQ, and thanks to the Apache license you can distribute the NATS server together with your commercial application.