The new Yahoo! Mail Beta looks a lot like the new Windows Live Mail Beta (new Hotmail)!
It all fits in one screen, its got the smaller font size, the right-click menus, drag and drop messages, the Colours and Help funtions at the top right, reading pane etc... Yahoo! have basically stolen the whole theme!
No colours to change on Yahoo! Mail Beta, though :( However, sign-up for Yahoo! Mail Beta is free and instant! No need for things like 'invitations' or whatever (like you need for Windows Live/Hotmail)... and Yahoo! also have this new 'tabbed' interface so you can have more than one email open in one go! The only other thing bad about it is it's quite slow!!!