Trillian

Trillian is a multinetwork chat client that currently supports mIRC, AIM, ICQ, MSN, and Yahoo Messenger. It supports docking, multiline edit boxes, buddy alerts, multiple connections to the same medium, a powerful skinning language, easy importing of your existing contacts, skinnable emoticons, logging, global away/invisible features, and a unified contact list. It has a direct connection for AIM, support for user profiles, complete type formatting, buddy icons, proxy support, emotisounds, encrypted instant messaging to ICQ and AIM, AIM group chats, and shell extensions for file transfers.