Trillian rocks the IM house. The most complete 3rd party IM client I’ve ever used, it allows simultaneous connections to AIM, ICQ, Yahoo, and MSN IM accounts, and also includes an IRC client. It auto imports your buddies from your local machine and AOL and Yahoo’s servers, so you can install it and go.
While it lacks a few features I seldom use (file transfer and chats with more than 2 people), it adds features I’ve always wanted in AIM. There is logging and you can rename users in your buddy list. Those two features alone make it more useful than AOL’s own client. Also, having a consistent interface among all services makes the UI predictable and easier to use.
I hope AOL, Yahoo, or MSN don’t try and block connections from Trillian clients, because after using it for just a couple hours, I’m hooked.