After my wife has switched from Hotmail to Gmail, she asked me to find a way to get notified new mail without opening the browser. Hotmail had a notification feature through Messenger application for Mac.

For Gmail, Google Notifier for Mac is the solution.

Once installed, it automatically checks new e-mail on Gmail. But the problem is that it checks new mail only in every 1/2 hour. To change the check interval, follow the steps below.

1. Click the Google notifier icon on the menu bar
2. Hold down Option and Command, and click the "Preferences..." menu."
3. A dialog with Key and Value fields show up
4. Enter "AutocheckInterval" to Key field
5. Enter the number of minutes you want as check mail interval.
6. Click 'Set' button
7. Close the dialog
8. Quit Google Notifier
9. Start Google Notifier

Now, the Google Notifier checks e-mail in the set interval.