I just found this while digging around on the OS X downloads section of Apple.com. Mail Unread Menu 2.3 shows your unread mail in an icon in the top menu bar. it works similar to Google Mail alert, and rather than just show you the red icon in the dock when you have mail in your inbox, it also alerts you when you have mail that's been automatically directed to other boxes using rules. When you click on the icon, it will tell you what mailboxes the new mail is in and you can go straight to that mailbox. Pretty handy! See more info here.