I have been using Trillian Pro since beta, and it hands down is the best chat client available. It incorporates four of the most widely used clients (AIM, MSN, ICQ, and Yahoo!) into one powerful, yet easy to use application. You can have all of your contacts in one convenient list, regardless of the service they are on. The lists are easy to sort and manage, and additionally show if you have mail (for AOL, MSN, Hotmail, and Yahoo). Top that off with some excellent plugins (I use one for showing news headlines for various sites, another to add in my POP mail accounts, two more that cover the weather and time for locations around the world.)

Trillian does offer a great free client (currently Trillian 0.74), however the Pro client is THE way to go, with great skins, plugins, and a more polished UI.

Gripes? I really only have two... the multiple profiles feature is a bit lacking, and somewhat cumbersome if you have more than 2 people using one machine with multiple Trillian profiles. And it is OS specific - Windows only. I do run it under VirtualPC on Mac OS X, however that is a poor solution, and still leaves me with my Linux box to fill the void. Of course, the OS comments don't reflect at all on the quality of the product itself, as many software developers currently only develop for one platform, and they have made a top-notch client for Windows.