Mac users need not feel left out, as Adium is there to support their IM needs. Like Pidgin, Adium is a free, open source chat client with a wealth of features such as tabbed chat, a combined contacts system, file transfer, and more. Lightweight and highly customizable, Adium also features extensions for added customization.
Editor's Note: Again, like Pidgin, Adium doesn't have voice or video chat support. Still, its interface is well done, and again, highly customizable.