Mail.Appetizer is a great little freeware plug-in for Apple Mail users. It displays a discrete translucent notification of incoming emails. The 1.0 version omits spam and only shows your inbox items after all filtering rules have been applied. The 1.1 beta allows you to select specific mailboxes.
Try it. It seems to play well with my multifarious Mac OS X Panther set-up. It's unobtrusive - you can adjust the transparency to your liking. And it's fun to see what's coming in as it comes in - just in case it's something good. If I'm expecting an email, I can pay closer attention and if not, I can just nominally track what's coming in.
Mail.Appetizer lets me catch important emails early without having to switch to Mail and look every few minutes. Since I sometimes forget to check, this is a good structure for me.
Without going into Mail, you can manage your mail on the fly as time and inclination permits: Click the subject to open the message in Mail. Control-click to mark the message read or get tough and shift-control-click to delete it.
Other Niceties:
The notification window is resizable to your taste.
You can specify the font and size of the type in the notification.