This is test.mosquitto.org. It hosts a publicly available Mosquitto MQTT server/broker. MQTT is a very lightweight protocol that uses a publish/subscribe model. This makes it suitable for "machine to machine" messaging such as with low power sensors or mobile devices.
The server listens on ports 1883, 8883, 8884 and 8885. Port 1883 is the standard unencrypted MQTT port and can be used with any MQTT client. Ports 8883 and 8884 use certificate based SSL/TLS encryption (TLSv1.2, TLSv1.1 or TLSv1.0) and require client support to connect. In both cases should use the certificate authority file mosquitto.org.crt to verify the server connection. Port 8883 allows unrestricted connections. Port 8884 requires clients to provide their own certificate to authenticate their connection. If you wish to obtain a client certificate, please get it touch.
You are free to use it for any application, but please do not abuse or rely upon it for anything of importance. You should also build your client to cope with the broker restarting.
If you have the mosquitto clients installed try:
If you do publish things to this server on a regular basis, please get in touch to satisfy my curiosity - there are lots of topics that look interesting but I know nothing about. I'm ral on the freenode #mqtt irc channel, or see the mosquitto source for contact details..