xmpp
Extensible Messaging and Presence Protocol (XMPP) (formerly named Jabber) is an open, XML-based protocol originally aimed at near-real-time, extensible instant messaging (IM) and presence information (e.g., buddy lists), but now expanded into the broader realm of message-oriented middleware. It was developed by the Jabber open-source community in 1999. Built to be extensible, the protocol has been extended with features such as Voice over Internet Protocol and file transfer signaling.
Unlike most instant messaging protocols, XMPP is an open standard. Like e-mail, it is an open system where anyone who has a domain name and a suitable Internet connection can run their own XMPP server and talk to users on other servers. The standard server implementations and many clients are also free and open source software.