Wikipedia:Tools/View source

// VIEW SOURCE addOnloadHook(function () {

 if (location.href.indexOf('viewsource=1') != -1) {
   sl = document.getElementById('wpSummaryLabel'); sl.parentNode.removeChild(sl);
   ed = document.getElementById('wpSummary').parentNode; ed.parentNode.removeChild(ed);
   return;
 }
 editbutton = document.getElementById('ca-edit');
 if (editbutton && location.href.indexOf('action=edit')==-1) {
   editlink = editbutton.getElementsByTagName('a')[0].href + '&viewsource=1';
   tabs = document.getElementById('p-cactions').getElementsByTagName('ul')[0];
   na = document.createElement('a');
   na.href = editlink; na.appendChild(document.createTextNode('view source'));
   li = document.createElement('li'); li.id = 'ca-viewsource'; li.appendChild(na);
   tabs.insertBefore(li,editbutton);
 }

});