A demo server is running on demo.flashmq.org. As an example, you can use the mosquitto clients and publish/subscribe with:
mosquitto_pub -h demo.flashmq.org -t topic/path -m "Hello $(date)" --cafile ./flashmq-ca.crt -p 8883
mosquitto_sub -v -h demo.flashmq.org -t '#' --cafile ./flashmq-ca.crt -p 8883
Active protocols are: mqtt TLS and non-TLS, and websockets TLS. The server’s CA certificate for the TLS mqtt port can be downloaded below. The websocket TLS port has a Let’s Encrypt certificate, so is accessible from any browser.