displays a transparent notification on top of your screen when new messages arrive. It shows only sender, subject and the first plain-text lines of the message, so you can determine whether the message requires your immediate attention. The prominence of the notification can be defined by adjusting transparency and display time.
Mail.appetizer is a plug-in for Apple's Mail and integrates seamlessly. It is dynamically loaded when launching Mail and you can change settings from within Mail's preferences.
Note that Mail.appetizer uses - as all Mail plug-ins do - several of Mail's private internal methods. Use it at your own risk.