Gmail Notifier Beta
Just about any Gmail user will want to this neat little freebie from Google. It automatically checks your Gmail account on a schedule you specify, then displays an icon alert in your system tray telling you when you've got mail. You'll also see the header of the message, and a snippet of the text. In order to read the mail, you'll have to open Gmail itself, so this isn't a mail reader. But for getting alerts that you've got e-mail in Gmail, it's ideal.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.