Yahoo have released a beta of their Yahoo Mail webmail interface to Yahoo Mail users, it asked me whether I wanted to try the beta when I signed in recently. It uses Ajax, meaning that the interface is very slick and doesn't need to reload the page to do most things. It looks like a response to Google's Gmail, which has led in the area of webmail interface usability for some time. It may also be a response to the Windows Live Mail Beta, Microsoft's up-and-coming version of Hotmail, which also uses an Ajax interface.
In my opinion, at the moment I prefer the Yahoo Mail Beta to Windows Live Mail Beta, it has a nicer-looking interface and it works in both Mozilla Firefox and Internet Explorer, where Windows Live Mail at the moment only works in IE.
The question is, will Gmail be rivalled by this new version of Yahoo Mail? Or even Windows Live Mail?