Sometimes one runs into a little free utility that just rocks. mail.appetizer is just one of these
. The link goes to the Tiger compatible beta version, for Panther users, there's a final version 1.0 (I'm not sure whether the newer beta also runs on earlier versions).
What it does is simple, elegant and great. You keep Mail.app in the background, and when a new message arrives, it's shown you in one of those almost-black but see through windows, where you can read the beginning of it (length depends on your preferences) and then mark it as read, open it in Mail.app, delete it or just click it away. I've definitely needed this. I'm sure you already knew about this and just didn't tell me. :P