XMPP
From wikipedia:
Extensible Messaging and Presence Protocol (XMPP) is an open-standard communications protocol for message-oriented middleware based on XML (Extensible Markup Language).[1] The protocol was originally named Jabber,[2] and was developed by the Jabber open-source community in 1999 for near-real-time, extensible instant messaging (IM), presence information, and contact list maintenance. Designed to be extensible, the protocol today also finds application in VoIP and file transfer signaling.
Unlike most instant messaging protocols, XMPP uses an open systems approach of development and application, by which anyone may implement an XMPP service and interoperate with other organizations' implementations. The software implementation and many client applications are distributed as free and open source software.