Empathy is a messaging program which supports text, voice, and video chat and file transfers over many different protocols. You can tell it about your accounts on all those services (it supports all protocols supported by Pidgin) and do all your chatting within one application.
Digsby is a multiprotocol IM client that lets you chat with all your friends on AIM, MSN, Yahoo, ICQ, Google Talk, and Jabber with one simple to manage buddy list.
Trillian is a chat client that makes it easy to support chatting on Astra, Windows Live, Facebook, Yahoo, MySpace, AIM, Email, Google Talk, Skype, ICQ, Jabber, IRC, Bonjour.
Kopete is an instant messenger supporting AIM, ICQ, Windows Live Messenger, Yahoo, Jabber, Gadu-Gadu, Novell GroupWise Messenger. It is suitable for personal and enterprise use.
ooVoo is a tool that allows you to add up to 6 participants to your video call, video call in high resolution, record video and audio calls, or share your desktop.
Palringo is described as a convenient as well as helpful ultimate Instant Messaging client for mobile phones and computers that lets you chat with all your buddies, no matter what IM they use, all from one universal Instant Messenger:
