Just found an fun new webapp called Wirify. Basically, you save it to your bookmarks, load a page you would like to see in wireframe, and click the link. It then gives you a wireframed version of your site.
Sadly, it doesn’t get it quite right for most of my sites. It’s based on jquery, so there are a few issues in how it sees components placed. For example, http://allaboutchris.co.uk: