Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.
Empathy - Apps/Empathy - GNOME Wiki!
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
Gajim - Full featured and easy to use Jabber client