I am sure a lot of people have heard of GMail by now. If you haven't, shame on you. I love how fast it is, I love the labeling mechanism they have and above all, the mount of storage space at your disposal. It's grown from about 1GB to 2.4GB since it first started. I am using about 10% of it with almost 33,000 mails. Searching for a particular mail is quicker than you can blink. The reason for being so fast is that is doesn't fetch all the HTML code from the server but renders all the code on the client. The mail data is fetched as a big XML file and parsed by a big JavaScript. This makes it a LOT faster when compared with other Web mail clients such as HoTMaiL or Yahoo!.
I am not sure if you still need to be invited to get an account, but if you're interested leave a message on this blog and I'll send you one. Be sure to obfuscate your email address a little, otherwise you'll end up being spammed to death.