The eXtensible Messaging and Presence Protocol (XMPP) is an open Instant Messaging protocol. An Instant Messaging (IM) server reports a users connection status (similar to Finger query), and allows the connected users to send short text messages back an forth between themselves (similar to an imediately delivered and opened email). The service predates, but is almost identical to, the popular cell-phone feature known as "texting". Server access requires using a XMPP client. Just copy & paste the full IP address into your client (the URI to precede the address is "xmpp:"). You must have an account on an XMPP server before you can IM other usernames, and each host has their own method of registration (some even allow "in-band" registration, where you create a username for yourself right on the server without first contacting the admin). Below are addresses of publicly open XMPP servers on which registered access is freely given.
[IANA Port assignment = 5222]

XMPP Servers