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.