If I were doing this, I would write a quick Perl script that uses Gungho for crawling your site, HTML::TreeBuilder for extracting the text, and Text::Aspell for spell-checking the page.
But, I probably wouldn't do this, because spell checking is irrelevant. It won't tell you if your prose makes sense, it won't tell you if you've used the wrong words, and it won't tell you if the content is out of date. It's best to go through your site and see if it all makes sense. Only a human can do that.