Gmail is also a great demo of Web 2.0 / Ajax in action. I really like it and think it is a step in the right direction for the web in general. A lot of websites are using it now, including these forums. It's built in to the software in various places.
I personally don't know how it works yet because I haven't had the need to use it on my personal stuff but I'd like to learn the basics in the near future since it seems everything is shifting that to web 2.0 now.
Windows Live Mail is pretty nice, and they finally got it working with both IE and Firefox. Right-clicking is probably my favorite feature, and everything seems to be right there. My main problem with it is the amount of spam I get under that account. Of course, Gmail will probably stay my primary account; I just have too much invested into it, and don't have as many problems with spam.