Empathy - Apps/Empathy - GNOME Wiki!
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
Facebook Messenger - Facebook Messenger is a faster way to message.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Digsby - Tagged makes it easy to meet and socialize with new people through games, shared interests, friend suggestions, browsing profiles, and much more.