spellr.us is a website tool that automatically spell checks websites and provides a monitoring service that regularly checks your website for spelling mistakes. It's in closed beta at the moment, but you can request an account and they come through fairly quickly. I played with it a little and it's very fast. It pulls up errors which it lists by error, rather than by page initially, and offers to show you your pages with all the errors on them. The language dictionary is US, but you can add to 'My Words' which is logical. Wasn't too sure about some of the terms they picked up - they didn't like weblog, blog, internet (lower case I think), webpages, podcasts, favourites (UK spelling), spyware and a few other technical terms.
I was impressed with the speed and ease of use. The display of results was fine, but I'd have liked some more native understanding of internet related terms. Would I pay for it? (They're looking at pricing models at the moment) No, not a chance.