Use Word's spell checker from .net
I was looking for a component like this and this is better. The only problem is that I have to install Word on the server.
Not sure if it's a good idea.
Here is a simple wrapper object that shows you how to late bind to Microsoft Word's spell checker from .net.
Use it like this:
MSWord word = new MSWord();
assert( word.CheckSpelling("antidisestablishmentarianism") );
Late binding from .net in C# to COM is not very straightforward, or documented....