Trillian is a popular multi-protocol IM client for not only Windows but even iPhone and Web now. With the new Trillian Astra which is the 4.0 version of Trillian, the IM client has drastically improved with the interface, usability and features. Its my preferred IM client for Windows. Finally after a long way of development and testing, Trillian is now available for the Mac! So you can have the seamless integration among all your IM services on your Mac.
Here are a few screenshots of the Mac version of Trillian: