ACE is web based code editing tool which provides a ton of editing functionality as well as supported languages. In addition you're able to use different themes for syntax highlighting. If you currently use the out of the box Joomla plain text editor this ACE integration might be worth looking at.
Version 0.0.5 adds spell checking. At least a kind of. It's not quit perfect but should already prevent some typos.
I had trouble getting this extension to work at first, but that was due a stupid mistake on my part. If you like to edit the html without a WYSIWYG editor then check this one out!