I have created a handy little bookmarklet that you can use to do the same thing.
This code has been tested in IE7, Firefox and Chrome and works well in all three.
Of course, it goes without saying that you don't actually edit the site, just the content that you see on the screen. Hitting refresh will remove any editing that you have done. This little tool is ideal if you want to see what different content would look like on the page without having to edit HTML. It can also be used to create spoof pages with false content.