If you have used Outlook 2003 for email, you will inevitably have encountered the new email notification pop-up window. It’s one of the better GUI design Microsoft has came up with and the transparency option makes it easy on the eyes too.
Now similar notification can be added to OS X’s Mail.app with Mail.appetizer. Give it a try, I like it so far. Though I would like to see options to set the border/margin size of the window. It is a bit thick for my liking.