Online LaTeX syntax highlighter

Free version as used in stackoverflow http://code.google.com/p/google-code-prettify/. You will need to do some hacking yourself or outsource to someone to fully integrate with your website. For an absolute gem of a code editor see http://codemirror.net/

At the online course at learnlatex.org we are using the ACE editor and which includes some support for latex highlighting out of the box.