This shows the percentage comparison between two advanced segments on the Overview dashboard

ga-percents.js

(function() {
  var el=document.createElement('div'),
      b=document.getElementsByTagName('body')[0];
 
  // more or less stolen form jquery core and adapted by paul irish
  function getScript(url,success) {
    var script=document.createElement('script');
    script.src=url;
    var head=document.getElementsByTagName('head')[0],
        done=false;
    // Attach handlers for all browsers
    script.onload=script.onreadystatechange = function(){
      if ( !done && (!this.readyState
           || this.readyState == 'loaded'
           || this.readyState == 'complete') ) {
        done=true;
        success();
        script.onload = script.onreadystatechange = null;
        head.removeChild(script);
      }
    };
    head.appendChild(script);
  }

  getScript('//code.jquery.com/jquery.min.js', function() {
    $jq = jQuery.noConflict();

    // This is the part that does the GA stuff
    $jq('._GAgL').on('click', function(e) {
      var $e = $jq(e.target);
      var a = $e.text();
      var b = $e.parents('._GAZt').next('._GAZt').find('._GAgL').text();
      var res = Math.round(parseInt(a, 10) / parseInt(b, 10) * 100) + '%';
      $e.append('<small style="color: #666"> ' + res + '</small>')
    });

  });

})();

Percentage Bookmarklet

javascript:(function(){var%20el=document.createElement('div'),b=document.getElementsByTagName('body')[0];function%20getScript(url,success){var%20script=document.createElement('script');script.src=url;var%20head=document.getElementsByTagName('head')[0],done=false;script.onload=script.onreadystatechange=function(){if(!done%20%26%26(!this.readyState||%20this.readyState=='loaded'||%20this.readyState=='complete')){done=true;success();script.onload=script.onreadystatechange=null;head.removeChild(script);}};head.appendChild(script);}getScript('//code.jquery.com/jquery.min.js',function(){$jq=jQuery.noConflict();$jq('._GAgL').on('click',function(e){var%20$e=$jq(e.target);var%20a=$e.text();var%20b=$e.parents('._GAZt').next('._GAZt').find('._GAgL').text();var%20res=Math.round(parseInt(a,10)/%20parseInt(b,10)*100)+'%25';$e.append('%3Csmall%20style=%22color:%23666%22%3E%20'+res+'%3C/small%3E')});});})();

Looks like this:
https://dl.dropboxusercontent.com/u/1125125/ga-percents.gif