I find this a nifty little application which is great for those who choose to hide their Dock. If you do hide your dock, there's no way to know if you have new mail unless you mouse-over the dock.

This puts a little red icon on my status bar.

I really miss using Mail Appetizer (a great pop-up notification that let you read, reply an delete new mail messages); unfortunately, it doesn't work with Leopard. (And, as far as I know, neither does Growl...