Extensible Messaging and Presence Protocol (XMPP) is an open-standard communications protocol for message-oriented middleware based on XML (Extensible Markup Language). The protocol was originally named Jabber, 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.
This article list several XMPP clients and servers.
Read Prosody for more information.
Available as community/ejabberd package
jabberd2 is an XMPP server, written in the C language and licensed as Free software under the GNU General Public License. It was inspired by jabberd14.
Read Jabberd2 for more information.