Trillian is a multi-network chat client that currently supports IRC, AIM, ICQ, MSN, and Yahoo! Messenger. It supports all of the features you've come to expect from a good IM client, including but not limited to docking, multi-line 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. Trillian is completely free with no nag screens or built-in ads.