XMPP is the standard for real-time communication. There is also xmpp-im implemented on top, that is used in Jabber and GoogleTalk, but you can also use XMPP for other ends, such as PubSub.

Clients:

• Adium X (MacOS)
• Pidgin (Cross-platform)
• Psi (Cross-platorm) – My favorite for dev purposes
• Slick (Symbian and Windows Mobile)
• Octro-Talk (Symbian and Windows Mobile)
• Google Talk (Windows and GTalk only)

Servers:

• OpenFire (Java)
• eJabberd (Erlang)

Resources:

• My personal motivation for Portuguese people to change to XMPP
• My notes on implementing a XMPP server and client
• Binary XMPP – Useful when in Mobile Phones