Google has added an unread message icon to Gmail.
“When you’re visiting sites other than Gmail, it’s easy to find out how many unread messages are in your inbox by glancing at the title of your Gmail tab or window. However, if you have a ton of tabs open, or if you use Chrome’s “Pin Tab” feature that hides everything except the tab’s icon, it can be tricky to figure out without switching tabs.”
Google says that this feature will only work Chrome 6 and above and Firefox 2 and above. In other words, everyone who will care about such a thing should be able to use it. To enable it, go to the Labs area in Settings and turn it on and save.