The Yahoo! mail beta is open to the public today. If you are a Yahoo! mail user, I recommend this upgrade. It offers much of the same functionality as Microsoft Outlook or Outlook Express. This isn't a Gmail-killer, but it is a huge advancement for webmail. Gmail and other services will no doubt feel the pressure and improve their AJAXiness.
I've been on the beta for several months and really like it over the old HTML/static Y! mail. Give it a try and don't forget to right-click.
Go to: Yahoo! mail