I’ve been using meebo as my IM client for a few months now. Like Trillian, Pidgin, and Digsby, meebo allows you to access all of your IM accounts through a single interface. They’re integrated with AIM, Yahoo!, MSN, Google Talk, ICQ, Jabber, Facebook, and MySpace.
It’s pretty amazing what they’ve managed to do in a web app. You can move, resize, and minimize your chat windows as fluidly as with a desktop app. Hitting the esc key even closes your active chat window, which is a really nice touch.
The only thing it can’t really do is tab between active chat windows using the keyboard.
The primary reason I use meebo, however, is that I never lose a message. Since you’re signing on to your different accounts using a web app, your IM accounts don’t log you out if you access it from another interface. That means you can stay logged on at work and at home and receive all your incoming messages. If you use the meebo notifier, you don’t even have to be signed in via a browser. The notifier also gives you one-click access to meebo, notifies you of events, and tells you when you’ve got new mail.