I've found syntax highlighters that highlight pre-existing code, but I'd like to do it as you type with a WYSIWYG-style editor. I don't need auto-completed functions, just the highlighting.
As a follow-up question, what is the WYSIWYG editor that stackoverflow uses?
Edit: Thanks to the answer below, I found two that look like they might suit my needs: EditArea and CodePress
EDIT: See this question also:
http://stackoverflow.com/questions/379185/free-syntax-highlighting-editor-control-in-javascript