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
All protocols are active: mqtt SSL and non-SSL and websockets SSL and non-SSL.
The server’s CA certificate can be downloaded below.