Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more. (GPL)

aMSN is a free open source MSN Messenger clone. (GPL)

Coccinella is a free and open-source cross-platform chat client with a built-in whiteboard for improved collaboration with other people. (GPL)

emesene is an open source instant messaging client, and is a "clone" of Windows Live Messenger. (GPL)

Empathy consists of a rich set of reusable instant messaging widgets, and a GNOME client using those widgets. (GPL)

The goal of Gajim is to provide a full featured and easy to use Jabber client. (GPL)

Kadu is an open source gadu-gadu protocol instant messenger client for Linux, BSD and Mac. (GPL)

Konversation is a graphical Internet Relay Chat client (IRC). (GPL)

Kopete is a multi-protocol, free software instant messaging client which integrates well with KDE. (GPL)

Miranda IM is an open-source multi protocol instant messenger client for Microsoft Windows. (GPL)

Pidgin is a multi-protocol Instant Messaging client that allows you to use all of your IM accounts at once. (GPL)

Psi is a free instant messaging application designed for the Jabber IM network (including Google Talk). (GPL)

qutIM - is new instant messenger. Main goal is to create fast and user friendly ICQ analogue client for Linux OS. (GPL)

SIP Communicator is an audio/video Internet phone and instant messenger that supports some of the most popular instant messaging and telephony protocols. (GPL)

Spark is an Open Source, cross-platform IM client optimized for businesses and organizations. (GPL)

Synapse is a new innovative instant messenger for the Linux desktop. (GPL)

XChat is one of the most popular IRC clients for Unix-like systems. (GPL)