Incredible. A webmail client with drag and drop, auto-complete and just everything what fat desktop email clients do. Fast.
How do they do this? No client-side Java or ActiveX-Controls, they only use things that are built into IE 5.0 and later, i.e. DHTML. Behind the scenes they use SOAP to talk to their server. I'll try to investigate more.
This is an interesting site about cutting edge DHTML: WebFx.