Instant messenger - libpurple

What is libpurple?
libpurple is intended to be the core of an IM program. When using libpurple, you'll basically be writing a UI for this core chunk of code. Pidgin is a GTK+ frontend to libpurple, Finch is an ncurses frontend, and Adium is a Cocoa frontend.

Who uses libpurple?

Adium - A user-friendly graphical IM program for OS X. Cocoa frontend. Adium is a free instant messaging application for Mac OS X that can connect to Google Talk, AIM, MSN, Jabber, Yahoo, and more.

EQO - An IM program for mobile phones.

Finch - A text-based IM program that works well in Linux and other Unixes. A console-based IM program that lets you sign on to Google Talk, AIM, Jabber, MSN, Yahoo!, and other IM networks. It runs on Unixes. It uses GLib and ncurses.

Instantbird - A graphical IM program based on Mozilla's XUL framework.

Meebo - A web-based IM program.

Palm - Maybe used in the messenger on the Palm Pre?

Pidgin - A user-friendly graphical IM program for Windows, Linux and other Unixes. a graphical IM program that lets you sign on to Google Talk, AIM, Jabber, MSN, Yahoo!, and other IM networks. It runs on Microsoft Windows and Unixes. It is written in C and uses GLib and GTK+.

Telepathy-Haze - A connection manager for the Telepathy IM framework.

Spectrum - Open source XMPP transport/gateway.