Get incoming mail notifications
Desktop e-mail software includes signals to let you know when you have new mail, by playing a sound or displaying a pop-up notifier. Google's free Google Notifier does the same thing for Gmail. It runs on your desktop and lets you know when you have new mail.
Google Notifier displays a notice on your desktop when you have a new message in your Gmail in-box.
It can also set your default desktop e-mail program to Gmail, so if you click an e-mail address in a Web page or document, the action starts a new mail message in Gmail. And Google Notifier will warn you about upcoming Google Calendar meetings too.
Unfortunately, it can't be configured to show you only new Priority Inbox messages; it shows you all new messages in your in-box.
Use the Authentication icon for verified senders
This tip can save you from major security headaches: Gmail can be configured to show a special icon next to e-mail coming from eBay and PayPal to verify that the e-mail is legitimate and comes from the sender it appears to be from. This will help protect you from being phished. To activate the feature, go to Settings --> Labs and enable "Authentication icon for verified senders."
The Authentication icon helps you make sure you're not being phished.
Customize the user interface
Gmail Labs is the place where Google lets you enable experimental Gmail features. It has a number of goodies to customize the user interface and streamline mail reading. A few of my favorites that I recommend enabling:
Add a "Mark as Read" button to the row of buttons at the top of Gmail, so you can mark a message as read without opening it. Otherwise, you'd have to click the "More Actions" menu to do the same thing; I'm exhausted just thinking about the extra work entailed.
Add a "Send & Archive" button to the outgoing message composition window, to send your reply to a message and archive the conversation in one click. Before the availability of this feature, Gmail users had to send e-mail, then archive the conversation by clicking separately. How did we ever bear it?
Send and archive in a single click.
View multimedia content inline in your e-mail. Enable "Flickr previews in mail," "Google Maps previews in mail," "Picasa previews in mail," and "Yelp previews in mail" to view the content of links to those services inline in your mail messages. Similarly, Google Voice users will want to enable "Google Voice player in mail" to listen to their Google Voice messages in e-mail.
View YouTube, Google Maps and other multimedia content inline.