Gmailto

Gmailto redirects any mailto: link on a Web page to your Gmail account instead of Apple's Mail client or other email program.

Move it anywhere, open Mail.app's preferences and select "Gmailto (2)" as your "Default Email Reader" from the "General" section. What's New
Version 2.0 updated the Gmail URL. Requirements
PPC, Mac OS X 10.3 or later, Google Gmail account.