There are way too many spell checking options out there for the Web. I've seen complex server-side components and I've seen complex client-side components. Some require Microsoft Office to be installed (for the dictionary) and some require an ActiveX control to be downloaded. All of them share one trait -- they require effort on part of the developer ... except for one product ... IESpell.
Check it out. It's free for personal use and can be downloaded and installed by the user. It includes its own dictionary and fires up with a right-click - Spell Check when your mouse is over a Textbox or Textarea. Enjoy.