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.
Herald is a free download and requires Mac OS X 10.6 Snow Leopard.