Kadu

Kadu is an open source Jabber/XMPP and Gadu-Gadu instant messenger client for Linux, *BSD, Mac and Windows.

Free Open Source Mac OS X Windows Linux BSD

Coccinella

Coccinella is a free and open-source cross-platform instant messaging client with a built-in whiteboard for improved collaboration with other people.

Free Open Source Mac OS X Windows Linux BSD

Shout

What is it? Shout is a web IRC client that you host on your own server.

Free Open Source Web Self-Hosted

Tkabber

Tkabber is a Free and Open Source client for the Jabber instant messaging system. Its writen in Tcl/Tk, and works on many platforms (tested on Linux, FreeBSD, NetBSD, Solaris, MacOS X and Windows 98/2000/XP).

Free Open Source Mac OS X Windows Linux BSD

Messenger for Desktop

A simple & beautiful app for Facebook Messenger. Chat without distractions on any Operating System.

Free Open Source Mac OS X Windows Linux

AQQ Instant Messanger

AQQ is a Polish (but available in English) multiprotocol instant messenger with support of: AQQ, Jabber, Gadu-Gadu, Tlen and ICQ.

Free Windows

Jabiru

Jabber client for Android.

Free Android

Sim-IM

Sim-IM, Simple Instant Messenger, is a multiple protocol instant messenger for MS Windows, GNU/Linux, FreeBSD. Sim-IM supports Jabber/XMPP, Google Talk, ICQ and AIM (Oscar), Yahoo! Messenger and Microsoft Messenger (MSN) protocols.

Free Windows Linux BSD