When new messages arrive, Mail.appetizer displays a transparent notification on top of your screen. 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.
The idea is nothing new, and in fact, is one that I'm usually turned off by, but this little plug-in pulls it off quite well and I have found myself really enjoying it. The main reason I tend to shy away from this sort of functionality is because it requires me to tell my e-mail client to check for new mail every X minutes, thereby conflicting with my desire to check my e-mail from my mobile phone when I'm away from my computer; there would never be any e-mail on the server because I would constantly suck it down from my computer, and no, leaving the messages on the server is not an option.
Because I like the plug-in so much and plan to keep using it, I've had to get into the habit of putting my machine to sleep when I leave the apartment so that it stops checking for e-mail while I'm out. The problem with this is that I almost always have something downloading in the background and putting the machine to sleep obviously breaks it (yes, I realize that I could tell the e-mail client to stop checking every X minutes, but to do this every time I leave my apartment would be ridiculous).
What I really need to do is whip up some AppleScript to tell Mail.app to stop fetching mail when my PowerBook no longer sees my Bluetooth-enabled mobile phone. What I wouldn't give for more time in the day.
If you are looking for more Mail.app plug-ins, be sure to check out Mail.app plugs and suggestions, a pretty good repository.