Do you work on large number of tabs at a time? Gmail shows number of new mails or unread mails in tab but if you are on number of tabs then you may miss that and you will no longer able to know how many unread mails are there and If any new mail arrives you may miss it too.
Unread message icon is a nice lab feature which allows you to show number of unread mails with Gmail logo in the tab.See below snapshot :-
Nice na? It is showing 1 unread mail in my Gmail its easy to get this :-
Signin to your Gmail Account,
Click on Setting –> Labs,
Now Search for Unread message icon and click on Enable
Now just hit Save Changes button and that’s it you are done.
PS. You must be using Firefox 2 or above Or Chrome 6 Or above to use this feature.