AJAX based web services are all the rage. I urge everyone to check out what's out there. Here's a short list of what I use:
Meebo A multi-protocol messenger client that runs in your browser. Great for those of us that move from machine to machine, or need to use a foreign computer temporarily.
Gmail I can't say enough about Gmail. It seems to do so much well. I have begun the process of abandoning my Hotmail account. Gmail is still by invitation only, but I have plenty. Email me if you need one.
Customized Google Try to make your own custom Google page. It's fun! You can drag the elements around and re-arrange endlessly.
There's much much more. Use Google to search for AJAX applications. Microsoft seems to be playing a game of catch-up with its "Live" products, but they are still clunky, and don't have the Firefox cross-compatibility.