This is a modified version of the Bespin bookmarklet that supports text selection and has a fullscreen editor. Go to a page with unreadable code and press the bookmarklet. Select the code you want in the editor and press the bespin button that appears.
Create a bookmark with the following text as location: "javascript:(function(){var elem=document.createElement('script');elem.setAttribute('src','https://github.com/thorsteinsson/Bespin-bookmarklet/raw/master/bookmarklet.js'\);document.body.appendChild(elem\);}\)(\)".