Adium
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Xtras
Adium features a beautiful OS X interface...
...or something more expressive and wild
Choose from multiple message and contact list styles
Manage multiple conversations with draggable tabbed chats
Organize contacts in a single list; combine duplicates
Full support of status across multiple services
Integrated Growl notification support
Integrated searchable message history
Maintain presence on multiple services simultaneously
Support for emoticons and message formatting
Yes, you can get the
cow image
in the screenshots too.