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.
Who uses libpurple?
Adium - A user-friendly graphical IM program for OS X.
Finch - A text-based IM program that works well in Linux and other Unixes.
Instantbird - A graphical IM program based on Mozilla's XUL framework.
Meebo - (Discontinued.) Between 2006 and 2012, Meebo provided a web-based instant messaging application that allowed users to log in with their AIM, MSN, Yahoo!, XMPP, etc. accounts. This functionality was powered by libpurple running on Meebo's servers. Meebo also powered IM functionality on several social networks via "The Meebo Bar." The functionality was based on XMPP and used libpurple running on Meebo's servers.
Palm - Maybe used in the messenger on the Palm Pre?
Pidgin - A user-friendly graphical IM program for Windows, Linux and other Unixes.
Spectrum - Open source XMPP transport/gateway.
Telepathy-Haze - A connection onalmanager for the Telepathy IM framework.