Gmail

Google is the king of webmail. When Gmail was invititaion-only, people were literally buying and trading invitiations for as much as $100.

In addition to the over 2.5GB of storage currently available (which keeps going up), the interface has revolutionized what we now expect from a web application. It is the single biggest precipitator of the flood of AJAX applications that are coming out. Many, many people prefer its web interface to their desktop email client.

In addition to it's outstanding interface, it also has many features that make it a must-use.

Message "labels" or tags
Robust email search via a Google interface
Customized 'From:' addresses
Multi-language (38) support
Rich text formatting
Interfaces with Picasa to send photos
Interfaces with Google Talk to check or send email
Free POP access and automatic forwarding
Import Contacts from Outlook or Yahoo! Mail
Automatic Custom Signatures
Both Yahoo! Mail and Microsoft Hotmail are playing catch up. Expect to see much improved versions of their email services by the end of the year.

http://mail.google.com/mail/