It’s all Ajax-y…
“The new Yahoo Mail Beta is touted as being as functional as a desktop email client (such as Outlook). Other new features include an integrated calendar timeline (including mashups with Yahoo Maps), drag and drop e-mail organization, message preview, tabs for messages, plus an integrated RSS reader. There are also plenty of links to other Yahoo properties, such as this one just above the inbox: “Start your own blog here” (which leads to Yahoo360).”
Read/Write Web has a good overview.
You can try it out now at Yahoo Mail.