XMPP
Originally known as Jabber, XMPP is built on XML and facilitates the exchange of structured data between multiple messaging platforms. In a nutshell, XMPP enables cross platform instant messaging. Apart from IM, XMPP is also used for transferring of files, VoIP, gaming, etc. Since XMPP doesn’t have a central master server, users can run their own XMPP server.