Growl notifications with Mail.app?
You need to download GrowlMail, however every time MacOS X is updated the version of GrowlMail becomes outdated and won't run (under Snow Leopard, at least). 

I think that the current version of GrowlMail doesn't work with Mac OS X 10.6.7 at present. Details of why it keeps breaking are here. It is annoying but seems unavoidable.