From Rudy Richter:
GrowlMail is a plug-in for Apple Mail that posts a Growl notification every time new mail arrives containing the text of the message.
Example notification for a new mail.
Quit Mail Open the GrowlMail package that you will find in the Extras folder of your download. Restart Mail.
You will find the preferences in a new tab of the normal Mail preferences (see image). You can disable GrowlMail, or disable notifications for mails marked as junk by Mail. If desired you can enable or disable notifications for specific email-accounts.