This neat little lab, created by Manu C, adds a small numerical counter to the Gmail icon visible on the browser page tab. This way, even if you are working on another website in another tab, you can quickly take note of whether or not you have received any new mail in your inbox. Please note that, according to the description, this lab is not available for all browsers. If you use Google Chrome, Firefox, or the Opera web browser then you can use this lab.
To enable this gadget on your account, go to the Labs tab in Gmail Settings and search for “unread.”