Instant Messaging(IM)

Instant messaging (IM) is a form of real-time direct text-based communication between two or more people using shared clients. The text is conveyed via devices connected over a network such as the Internet.

ref:

Instant Messaging - http://en.wikipedia.org/wiki/Instant_messaging

Extensible Messaging and Presence Protocol(Jabber) - http://en.wikipedia.org/wiki/Jabber

Instant Messaging Protocols -

http://en.wikipedia.org/wiki/Comparison_of_instant_messaging_protocols

Pidgin (software) - http://en.wikipedia.org/wiki/Pidgin_(software)

A free chat client used by millions. Connect easily to MSN, Google Talk, Yahoo, AIM and other chat networks all at once

Pidgin (Source Code) - http://pidgin.im/download/source/

libpurple - http://developer.pidgin.im/wiki/WhatIsLibpurple

Libpurple has support for many commonly used instant messaging protocols, allowing the user to log into various different services from one application

SourceForge projects that use libpurple -

http://sourceforge.net/search/?type_of_search=soft&words=libpurple

Meebo - http://en.wikipedia.org/wiki/Meebo

Meebo is an instant messaging program which supports multiple IM services, including Yahoo! Messenger, Windows Live Messenger, AIM, ICQ, MySpaceIM, Facebook Chat, Jabber, Google Talk, Myyearbook IM and XMPP; it is based on the free and open source library libpurple created by the software developers of Pidgin

Miranda IM - http://en.wikipedia.org/wiki/Miranda_IM

Miranda IM is an open source multiprotocol instant messaging application, designed for Microsoft Windows. Miranda is free software distributed under GNU General Public License