Yahoo is beta testing a new version of its free Web-based e-mail, and it's looking more like Microsoft Outlook and adding similar features. Think of it as a richer Web client that performs almost like a desktop client. Lots of people are familiar with the Outlook interface, and a free service with similar basic functionality and user controls makes the WebOS more real. Gmail, Hotmail and others will follow suit or come up with something better, but it's difficult to get away from the conventions (folders, windows, icons, etc.) that have been accepted since the Macintosh was introduced in 1984.
Yahoo Mail current (Compose mail)
Yahoo Mail beta (via Dave Winer) (Compose mail)
Microsoft Outlook (Two windows nudged together) (Compose mail)