One of the first labs I enabled was "Unread message Icon". Once enabled you can tell how many unread messages are in your inbox without browsing to your Inbox tab. This lab embeds the number of unread messages you have right into the Gmail icon itself, like this:
To turn it on: From your Inbox, click on the gear, then Settings. Now go to Labs and scroll down to Unread message icon, click enable and Save Changes at the bottom. Next time you have a ton of tabs open, just glance over at your Inbox tab to see if your unread mail count has changed.