I personally am quite a big fan of plug-ins for Mac and the ones I've tested for Apple Mail all seem to bring more to the table. This one is no exception.
Mail. appetizer shows all new incoming mail as a slightly transparent pop up window.
The plug-in is easy to install and can be configured from the preferences menu in Apple Mail.
We liked how you could set the level of transparency, the dismiss time for the window or how you could set message size and font. You can also choose what details appear in the notification window such as the header or the inbox it is destined to.
Mail. appetizer can also be set to every one of your inboxes or just the ones that you choose.
If Apple Mail is your default mail client on Mac, we suggest installing the Mail.