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:
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