Psi is a free, open source instant messaging application designed for the Jabber IM network (including Google Talk) compatible with Windows, Linux and Mac OS X.

If you're tired with the Jabber functionality of Empathy or Pidgin, both lacking lots of important features for this protocol, you should give Psi a try. Psi comes with full Unicode support and localizations, easy file transfers, customizable iconsets, and many other great features.

imo.im is a young web-based multi-protocol IM, co-founded by ex-Googler George Harik.

A major new release was recently launched that supports Skype (making it one of the only multi-protocol services to do so), and allows users to hold Voice and Video Chats on all the most popular instant messaging platforms (AIM, MSN, GTalk, MySpace, Yahoo, ICQ, Jabber).

The service is free, enables single-sign-on without download or registration, supports most major languages, and can be used on all the major web browsers and operating systems. It also has a pop-out mode that opens imo in a separate browser window. You can set imo to Pop-Out mode by clicking the "Pop-out" link located at the top-right corner of your screen, above your buddy list.

But the best part about imo is of course the voice and video support which i bet especially Linux users will find them as killer features.