Adium is a free and open source instant messaging application for Mac OS X, Mac OS X written with Cocoa API released under the GNU GPL, developed by the Adium team. Based on libpurple protocol library may, Adium can connect unlimited messaging accounts on any combination of supported messaging services (see further down the list) and chat with other people providing these services.
Adium is easy to install and configure, with a very active 'Xtras' website, there are literally hundreds of ways you can customize. There is no doubt: This program will help you keep in touch with friends and family in a way not just iChat. The biggest bummer? Adium does not support audio or voice chat so you have to use iChat or Skype for that stuff.