XMPP

Description

Homepage	http://xmpp.org/about/
Type	Open standard
Identity	Jabber ID (JID), e.g. usr@a.b.c/home with "home" being a "resource", which distinguishes the same user when logged in from multiple locations (simultaneously)
Interoperability	Google Talk, LiveJournal, Gizmo

Note that XMPP is special, due to being an open protocol. There is no central XMPP server, the structure is like e-mail. Everybody (with a static IP) can host an XMPP server, optionally linking it up to the global XMPP network (otherwise, only local users on the same server are accessible).

There are already several larger XMPP installations, like Google Talk, LiveJournal and Gizmo.

This openness also means that unless you want to install your own server (not recommended), you also have to choose a hoster for your account (just like for email). Besides the three mentioned in the previous paragraph, the community portal jabber.org also has a list.

More technical information can be found at the XMPP Standards Foundation.