Gmail.
So integrated into other stuff its great. Crapload of space. A quick, simple and really nice looking layout. Built in Google Chat service, thats freaking awesome if you have plenty of people who use Google Talk, or have a GMail account.
And the folks at Google have an awesome sense of humor. Those with Gmail accounts, have you ever noticed when you go into your SPAM inbox, and look at the RSS Feed bar at the top of all the emails in the box, it gives you "Great" Spam Recipes. As in the food spam. I dont know about ya'll, but I think thats hysterical.
If you have an address or shipping track code in an email, Gmail can see it and will have a built in link all ready to click and take you straight to that tracking codes site on FedEx, UPS, etc...
The little bar at the top left of the screen in Gmail quickly gets you to google.com, your Google Calendar, or your GMail screen if not already there.
Gmail has a very sweet setup for WebService. I believe its the best by far. They also offer POP3 access which many free services dont.
-C