
Proteus is a multi-protocol support all within a single application, offering support for AIM (including .Mac accounts), MSN, Yahoo, ICQ, Jabber, iChat Rendezvous, Gadu Gadu, and Sametime.
From the Defaultware website:
Proteus works seamlessly with Mac OS X by integrating Address Book, Cocoa and WebKit technologies, resulting in a powerful application that feels right at home on your Mac. By using Address Book, Proteus lets you easily display real-names instead of usernames in your contact list. As for Webkit, we are able to provide customized chat views that fit every personality.
And because we use Cocoa, we support all the niceties that it gives us for free, like customizable toolbars, spell checking, integration with system services, drag and drop (file transfers, text clipping) - all the stuff that makes the Mac experience legendary.
We even support UNIX technologies thanks to Mac OS X's BSD heritage, such as running shell scripts when someone messages you or displaying rich LaTeX equations!
Proteus 4.1 is a commercial application that is free to download and try, and $15 to register. There are downloadable add-on packs for emoticons, sounds, and icons.
http://www.defaultware.com/proteus/index.php