Checkout http://www.roundcube.net
It’s an AJAX based web mail application. It’s inspired by the Mac interface, is open source and is one of the best webmail systems I have seen. Quite simple, neat looking and highly usable.
And after you have seen this, take a quick look at http://www.rememberthemilk.com/. It’s a Web 2.0 style to-do application. Quite cool!
Thanks to Navin Dhanuka for sending the link.