Adium

Open Source, so everyone can see how Adium works and help improve it
Support for a wide range of different Instant Messaging services (see the full list)
A delightful UI
Tabbed chat windows
Mac OS X integration
Address Book integration
WebKit Message View: Theme your chat windows
Combined Contacts: Merge your contacts so that each one represents a person, not an account
A sophisticated events system (including Growl notifications)
OTR Encryption
File Transfer
Xtras and many, many other customization options
A beautiful icon, the "Adiumy" duck
Translations: Adium speaks 27 different languages