Yahoo Mail is currently in Beta testing. Apparently it is using AJAX to power the site meaning you don't have to wait for a page to reload in order to check mail, delete mail etc...
On the link below it says:
Quote:
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Look out, Outlook: Yahoo Mail is beta testing a new interface that brings the look and feel of a desktop client to Web mail.
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Here's some more info:
http://blogs.pcworld.com/staffblog/archives/000928.html
I'm keen to test it out

I'm guessing Hotmail will needs an upgrade!