Evan Shoenberg has built a new Leopard-compatible version of the plugin for Mail.app that shows new messages via the Growl notification system. It’s aptly named, GrowlMail. You can download the latest version here:
If you have Growl installed, you should be able to install this update, run Apple Mail, and see the GrowlMail preferences pane.
However you might find after installing Growlmail that you can’t see this pane. In that case, close Mail, and enter the Terminal app and run the following:
defaults write com.apple.mail EnableBundles -bool YES defaults write com.apple.mail BundleCompatibilityVersion -int 3
After that everything should work as expected.