Windows 7 only: Gmail Notifier Plus displays your unread email count right in the Windows 7 taskbar, including popup message previews and Jump Lists integration.
Once you’ve launched the application, you will be prompted for your Gmail account information—after which you will see the unread count as an icon right on the taskbar button and hovering your mouse will show a preview of your unread email. Right-clicking on the button uses Windows 7’s excellent Jump Lists feature—with quick and easy access to frequent tasks like opening your inbox or composing a new email message.
Gmail Notifier Plus is a free download for Windows 7 only—hit the link for more screenshots and the free-registration-required download, or use the mirror to just download it quickly. For more ways to add email notifications, check out Growl for Windows or Gmail Notifier for Ubuntu.