From Erik Hinterbichler:
Herald is a notification plugin for Mail. app, Apple's Mac OS X email client. Inspired by the excellent Mail. appetizer plugin, Herald shows you popup notifications whenever you receive new Mail. It also allows you to perform common actions directly from within the notification window, including deleting messages, initiating replies, viewing messages in Mail, or marking messages as read. Herald's appearance is highly customizable, allowing you to change the background color, text color, size, shadow, transparency, and position of the window. Herald even provides a scrolling text view so you can read full messages without having to switch back to Mail. Note: Mail plugins are not officially supported by Apple. While Herald has been thoroughly tested, it's possible it may cause unintended conflicts with Mail or with other unofficial Mail plugins, especially after OS updates. It's recommended that you backup your computer before installing Herald or any other Mail plugin, and always before installing any updates to Mail or OS X.