iGoogleBar is a firefox extension which adds Google Apps favicons to the Google Apps Bar and you can use them to see previews for the Google Apps. It adds more functionality to the Google’s services menu which appears on the top whenever you use any of the Google services like Gmail.

For example, if you’re using Gmail, you can see how many unread items you have in Google Reader, or you can click on the items for a preview of latest items.

It currently only have support for Calendar, Documents, Gmail, Groups, Notebook, Photos, and Reader. So clearly it doesn’t work with all Google services. It also takes a moment for the toolbar to load, so you might not notice it at first. And it is not very much customizable. But it’s still very cool none the less.