I've been looking at incorporating a spell checker into an ASP.NET application on and off for a while, but could never really find anything that was suitable - either there are popup pages involved, or it was all client side validation which meant dictionaries had to live on the client machine to be useful.

I've just run across an AJAX based spell checker at broken-notebook that looks the goods. It's in PHP, but I'm sure it won't take long to knock into shape as an ASP.NET page.

I'll post some results here soon.