Desktop notifications for email and chat are now available for all Google Apps domains. These are HTML5 desktop notifications which display pop-ups whenever a chat message or new email arrives.

Editions included:
Google Apps, Google Apps for Business, Government and Education

Languages included:
All languages supported by Gmail

How to access what's new:
In your Gmail “Settings”, go to the ‘General’ tab and scroll down to the new ‘Desktop Notifications’ section. Click on the link to opt-in to this feature. Once enabled, choose if you want notifications just for chat, email or both. You can also choose just to be notified for email in Priority Inbox if you have that enabled.

Note: This functionality is currently only available for people using Google Chrome, but Google is working to make notifications part of the standard Web platform.

For more information:
http://gmailblog.blogspot.com/2011/01/desktop-notifications-for-emails-and.html
http://mail.google.com/support/bin/answer.py?hl=en&ctx=appsfeed&answer=1075549