This is an old question, and here's an old answer that has once worked flawlessly for me for many years, but doesn't any more, at least not as of January 2016:
The "Generated Source" bookmarklet from SquareFree does exactly what you want -- and, unlike the otherwise fine "old gold" from @Johnny5, displays as source code (rather than being rendered normally by the browser, at least in the case of Google Chrome on Mac):
https://www.squarefree.com/bookmarklets/webdevel.html#generated_source
Unfortunately, it behaves just like the "old gold" from @Johnny5: it does not show up as source code any more. Sorry.