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:"). Below are addresses to various XMPP servers on which public access is freely given* (each host has their own registration webpage).
[IANA Port assignment = 5222]