Yahoo has started limited beta testing of their new web-based email application and it is a definite "wow". It feel very slick and behaves likes a desktop application. It has all the nice things like drag-and-drop, key-bindings, indexed searches as well as the features of the old interface such as filtering, spam blocking, etc. My opinion might be biased, but in all honesty I was impressed when I first saw what the mail team was working.
This blog entry gives some ideas with screenshots of what's in the new Yahoo! Mail.