MailCue

MailCue is the must have mail notifier for OS X. MailCue works with any IMAP provider, sitting in your menubar notifying you of incoming mail.

What separates MailCue from the competition, is the integration right into your Gmail or mail provider with such features as, reply, reply to all, forward, full html message previews including embedded images, account signatures, move a message, mark a message as spam, putting an account to sleep, and more.