I know this is an old post, but I just found this piece of gold. This is old (2006), but still works with IE9. I personnally added a bookmark with this.
Just copy paste this in your browser's address bar:
javascript:void(window.open("javascript:document.open(\"text/plain\");document.write(opener.document.body.parentNode.outerHTML)"))
As for firefox, web developper tool bar does the job. I usually use this, but sometimes, some dirty 3rd party asp.net controls generates differents markups based on the user agent...
EDIT
As Bryan pointed in the comment, some browser remove the javascript:
part when copy/pasting in url bar. I just tested and that's the case with IE10.