Automatically detects if a user is logged into gmail, and if they are, will replace all the mailto: links on your webpage with links to the gmail compose window (automatically filling in the To field):

gistfile1.js

$('<img/>').hide()
   .attr('src','https://mail.google.com/mail/photos/img/photos/public/AIbEiAIAAABDCKa_hYq24u2WUyILdmNhcmRfcGhvdG8qKDI1ODFkOGViM2I5ZjUwZmZlYjE3MzQ2YmQyMjAzMjFlZTU3NjEzOTYwAZwSCm_MMUDjh599IgoA2muEmEZD')
   .load(function(){
      $('a[href^="mailto:"]').each(function(){
         var email = $(this).attr('href').replace(/^mailto:/,'');
         $(this).attr('href','https://mail.google.com/mail/?view=cm&fs=1&tf=0&to='+escape(email));
      });
   })
   .appendTo('body');