Google has just added another useful feature in Gmail: desktop notifications for email and chat.
After enabling this, you will get instant desktop notifications for new mails and chat messages. Clicking on the notification will either open up the new mail in a new window or take you back to your Gmail tab. You can also adjust the way notifications work by clicking the wrench icon on the notification.
To activate this feature, click on the Settings link in the top right corner of Gmail and scroll down to the “Desktop Notifications” section.
There you can choose to get notifications of only chat messages, Priority Inbox or for all mails in your inbox.
This feature currently works only in Google Chrome, but Google has plans to bring it to other browsers that support HTML5, like Firefox 4 and Internet Explorer 9.
If you want desktop notifications for new mails in all folders and even after closing Gmail window check out Google Mail Checker Plus.