Adium is a free instant messaging application for Mac OS X, released under the GNU GPL. With Adium, you can connect to any number of messaging accounts on any combination of supported messaging services and then chat with other people using those services.
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aMSN is a free open source MSN Messenger clone, with features such as, Offline Messaging, Voice Clips, Display pictures, Custom emoticons, Multi-language support, Webcam support, Full-speed File transfers, Group support, Tabbed chat windows, plus many more.
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Traditionally, chat clients on the Mac have been anything but glamorous. Colloquy is an advanced IRC, SILC & ICB client which aims to fill this void. By adhering to Mac OS X interface conventions, Colloquy has the look and feel of a quality Mac application.
Pidgin is a multi-protocol Instant Messaging client that allows you to use all of your IM accounts at once. Pidgin is free software. It is licensed under the GNU General Public License (GPL).
X-Chat has two good features of usability: colored nicknames and alignment by text as opposed to alignment by nicks.