Well, there is one...
http://b2evo.astonishme.co.uk/google-spell-checker

BUT, it's not been updated for a while. Click on the red warning sign.

I use Firefox (spell check this field) and it does a pretty good job of catching my cockups

My Firefox also does spell checking, but it does it all the time and I don't recall installing an addon or having to ask it to do it. I wonder if it is a setting I told to always use?

coldhart just put a note in your footer that says something like "all words, even the ones you're not familiar with, are real words even if your crummy dictionary doesn't have them in it" and you won't need a spell checker. Everyone else will need a better dictionary, but you won't need a spell checker :)

and I don't recall installing an addon or having to ask it to do it

It was an addon that eventually became a core feature :)

FireFox does it for me too: plus there are lots of language packs that can be installed.

What is it that Firefox uses? aspell, myspell and/or hunspell.

Someone MUCH smarter than myself might be able to make use of these links:

http://hunspell.sourceforge.net/
[url=http://sourceforge.net/docman/display_doc.php?docid=29374&group_id=143754]Hunspell Manual[/url]

http://aspell.net/
[url=http://aspell.net/0.61/man-html/Writing-programs-to-use-Aspell.html#Writing-programs-to-use-Aspell]Writing programs to use Aspell[/url]