If you're using Google Toolbar 5 for Internet Explorer or Firefox, you can now add the Gmail custom button to get new message alerts, see previews of your mail and use Toolbar's search box to find any message no matter what page you're on.
Google Toolbar also has a “Send to” feature: when you click on the "Send to" icon, a Gmail compose window opens that automatically includes a link to the page you're currently on and any text you have highlighted on that page. This makes it easy to email your friends interesting pages you come across as you browse the web.
Plus, you can also add Google gadgets to your Toolbar to bring your favorite websites and online services closer to you. One of my favorites is the Google Calendar gadget: by clicking on the calendar icon, I can check out my schedule for the next few days and even add events. [Gmail Blog]
More info: Toolbar's gallery