Notifications can be very valuable to users, but there's only so much a developer can do with an icon's worth of pixels. Many Chrome extensions use browser actions as a notification area. As it turns out, web sites have a great way to deliver non-modal message like (see pic) with "notifications API," which was first introduced in Chrome 4 for Windows. Gmail Notifier extension was an early adopter of notifications.
As of Chrome 5, Google announced "that notifications are also available to extension developers. When notifications are used from an extension, there're no permission prompts or infobar warnings. The experience is seamless - it just works," Google writes.