The question might be better stated as "What syntax-highlighting editor do you recommended to replace an html textarea in my web app?" (Some of the other answers here deal with desktop apps or pure-syntax highlighters, not client-side editors)
I also recommend CodeMirror, it's written in Javascript and supports lots of browsers. It uses a real parser (rather than regexps) so it can deal with complex problems like correctly highlighting escaped strings. The developer is also very responsive on the discussion group.
