Pidgin, Finch, and libpurple
List of projects accepted into Pidgin, Finch, and libpurple
Pidgin is an instant messaging program which lets you log in to accounts on multiple chat networks simultaneously. It runs on Windows, Linux, and other UNIX operating systems. Pidgin is compatible with the following chat networks out of the box: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, MXit, Novell GroupWise Messenger, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. It is written in C and makes heavy use of GLib and GTK+.
Finch is a command line instant messaging program. It also lets you log in to accounts on multiple chat network simultaneously, and it is compatible with the same chat networks as Pidgin. It is written in C and makes heavy use of GLib and ncurses.
Pidgin and Finch utilize a programming library called libpurple in order to connect to the various IM networks. libpurple is also used by the OS X IM application Adium (although Adium is a separate project and not affiliated with Pidgin/Finch/libpurple). It is written in C and makes heavy use of GLib.