meebo is the AJAX (That's Javascript and XML) programmed, browser-based Instant Messaging God of the internet.
If you visit one website this year, there's probably no reason for you to have an internet connection. More to the point, that website should be meebo.com. You should bookmark it even though a lot of its usefulness comes from being able to use it away from your own computer.
What can you do on meebo? You can send instant messages to your friends. Why is this so great? Because it does what Trillian did, but better and more conveniently. You don't need to download anything, it doesn't hassle you for updates. When it does update, you don't even notice, you just log on to the website and it loads as usual.
I don't use MSN anymore, I don't use AIM anymore. Why would I? So I can experience the joy of a computer that is bogged down by a thousand windows opening whenever I start it up? meebo is my new favourite website.