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Most (if not all) WYSIWYG HTML editors like TinyMCE use a contenteditable container (e.g. div) and all current browsers offer spell check in the contenteditable element.

It actually is a TinyMCE problem. (Article says that Tiny used the non public URL and if it got back 500 error, just defaulted to "oh, no spelling errors then")

In TinyMCE you get both the browser-based spell checkers (via the right click context menu), and the spelling suggestions in TinyMCE (depending which spelling backend you're using: aspell, enchant, google spell, etc. http://www.tinymce.com/wiki.php/plugin:spellchecker)

We use TinyMCE and a .NET plugin that uses the GoogleSpell check service. It's dead right now and we are trying to find a solve for it.

I decided to use Yandex Speller since at the time I was researching this issue it was clear that Google Spell was an undocumented and unsupported feature. http://api.yandex.ru/speller/

According to the TinyMCE, you can use aspell. http://www.tinymce.com/wiki.php/Plugin:spellchecker

You can get a windows build of aspell at http://aspell.net/win32/

Instructions are on the wiki on how to wire it up.