(http://toolbar.google.com/gmail-helper/)

The Gmail Notifier is a downloadable Windows application that alerts you when you have new Gmail messages. It displays an icon in your system tray to let you know if you have unread Gmail messages, and shows you their subjects, senders and snippets, all without your having to open a web browser.

Although this program is in beta, it actually works quite well. Every 2 minutes, you can see a little motion in your system tray telling you that it's checking your gmail account for new mail. If there is new mail, the graphic in your tray changes to a picture with red instead of just yellow and a little popup tells you the info about the new message(s). Especially useful for when you're away from your computer and you don't want to check your email to find nothing there.