GMail Notifier for Google Apps is clone of the official GMail Notifier with one small difference - it works with GMail as well as Google Apps for your Domain.
When you new email messages, the notifier shows you the email subjects, senders email address and a short snippets, all without your having to open a web browser.
The GMail Notifier for Google Apps is not from Google but the developer is officially recognized as a Google Enterprise Partner.
Bonus Tip: Do you know that Google Talk can also be used as GMail Notifier for Google Apps though you'll have to sign out from your regular GMail account. I personally use Microsoft Outlook to check my GMail and Google Apps email.
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