Chrome: How to register Gmail as a default "mailto:" handler

1. First, open up a Gmail tab. You must do this from the Gmail tab, not your html5rocks one. :)
2. Open your javascript console (cmd-opt-j on Mac, ctrl-shift-j on Windows or Linux) and enter:
navigator.registerProtocolHandler("mailto",
  "https://mail.google.com/mail/?extsrc=mailto&url=%s",
  "Gmail");
Accept the confirmation from the browser.
3. Click this mailto: link to test out your new Gmail mailto hookup!

Original article by Paul Irish