I was asked today how to make GMail the default email client so that when you send email it will be automatically opened up. You use to be able to do this with the GMail notifier, but it looks like Google stopped developing it (no updates since 3/2006).
You can now do this with Google Talk (Google's IM chat client). Open the Setting dialog under General, check the "Open GMail when I click on email links", press the OK button.
This will make Gmail as your default mailto: handler. Which means when you click a hyperlink email address, a Gmail compose window will be pre-populated with you email address in your default web browser.