I really like GrowlMail but it isn't up-to-date with OS X. So here's an AppleScript alternative.
You need the growlnotify utility that comes in Growl Extras on the Growl installer disk image.
Create the following AppleScript and save it. Then create a match-all rule in Mail.app (Mail » Preferences » Rules » Add Rule) to run the script as the performed action. Don't apply it to the existing messages, just new ones.
using terms from application "Mail" on perform mail action with messages theMessages for rule theRule tell application "Mail" repeat with m in theMessages set sub to m's subject set sndr to m's sender set msg to sndr & " " & sub -- display dialog msg do shell script "/usr/local/bin/growlnotify -I /Applications/Mail.app -m " & quoted form of msg end repeat end tell end perform mail action with messages end using terms from
Mac OS X Hints
http://hints.macworld.com/article.php?story=201012041205380