MailBrew

MailBrew is an email notifier that supports Gmail, Google Apps email, and IMAP.
MailBrew is completely free.

It's an Adobe AIR application which means it runs on Mac, Windows, and Linux.
Main Features:
- Multiple accounts. Add as many email accounts as you want.
- Configurable notifications. Choose (per account) where you want your notifications to appear, and what sound you want them to play.
- Runs in the background. Close the main application window to keep it out of your way.
- Summary window. Open the miniature summary window to show you how many unread messages you have in each account.
- Highly configurable. Use the default preferences, or configure things like the update interval, notification display length, idle period, and much more.