I've just tried spellr.us for the first time. It's in limited beta, and is all about spell-checking; you enter a link, wait for a while ((Which of course differs, depending how big a site in question is.)) and then get a detailed report that tells you what's up.

The site differs between possible and probable spelling errors, and lets you see them through an overlay map of the site you've defined. Future releases will most probably let you download a PDF version of the report.

It took me a mere four minutes to have my entire blog checked for errors, and while I (thankfully) didn't find any direct errors, it was interesting to see what words I was using that might confuse readers. The company responded quickly to my feedback, which was nice to see. Remember that the site is in private beta; it took me appx. two weeks to get an invite, and the system apparently doesn't let me invite others.

I've wrapped it all up in a screencast which is viewable in flash right here.