This plugin makes the ACE editor (http://ace.ajax.org) available for editing content in Joomla.
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.