Bookmarklet to resize a website's content to a centred, 960px-wide main column.

960this!.js

(function() {
  if(document.querySelector('#wrap960') ) return;
  var content = document.body.innerHTML;
  var wrap = document.createElement('div');
  wrap.setAttribute('id','wrap960');
  wrap.setAttribute('style','position:relative;width:960px;margin:auto;');
  wrap.innerHTML = content;
  document.body.innerHTML = '';
  document.body.appendChild(wrap);
})();

/* Minified */
(function(){if(document.querySelector('#wrap960'))return;var content=document.body.innerHTML;var wrap=document.createElement('div');wrap.setAttribute('id','wrap960');wrap.setAttribute('style','position:relative;width:960px;margin:auto;');wrap.innerHTML=content;document.body.innerHTML='';document.body.appendChild(wrap);})();

To use, add the code below to a new bookmark:
javascript:(function(){if(document.querySelector('#wrap960'))return;var%20content=document.body.innerHTML;var%20wrap=document.createElement('div');wrap.setAttribute('id','wrap960');wrap.setAttribute('style','position:relative;width:960px;margin:auto;');wrap.innerHTML=content;document.body.innerHTML='';document.body.appendChild(wrap);})();