Adium
Adium is an instant messaging client for Mac OS X supporting multiple protocols through the libgaim library. It is written using OS X's Cocoa API, and it is released under the open source GNU General Public License. Adium was originally created by a college student named Adam Iser, and the first version, 1.0, was released in September 2001.
The latest official 1.x release of Adium -- not making use of libgaim, and limited to the AIM protocol -- was version 1.6.2c in September 2002; work on 1.x mostly stopped with the beginning of the development of a new version. This version, re-named from Adium 2.0 to Adium X, is a complete rewrite of the code, and implements such features as support for more IM protocols (such as ICQ, MSN Messenger, and Jabber) using libgaim. Most of the work on libgaim itself is done by the gaim team; the Adium team mostly works on the Adium X GUI. Adium X, however, is still in its alpha stages, although the team usually hands out new pre-releases biweekly, showing off the progress they have been making.
One aspect that didn't change between Adium 1.x and Adium X is the massive use of modules: Most of Adium's functionality comes from plugins; the core program by itself does very little. The plugin system will allow users to pick and choose which features they want to include in the program.