I have been searching for something that does this for a while now, but with no luck yet... We have an internal application that stores/uses text in HTML format. Now I am looki
Since I was too much of a cheapskate to buy a spell checking control for our internal app (that would have been total overkill), I cobbled together an IExtenderProvider that does wavy red line spellchecking on textboxes. I've been sitting on the code for a while but I just blogged about it; there's a sample application there.
Some caveats:
But, hopefully it'll give you some ideas, even if you're not able to use it directly.
Another thing you might consider is that I know some of the WPF text controls have spell checking built in. But I'm not sure how they handle HTML.