CodeMirror [↗]
CodeMirror is a versatile text editor implemented in JavaScript for the browser. It is specialized for editing code, and comes with a number of language modes and addons that implement more advanced editing functionality.
I came across this today whilst searching for some information about syntax highlighting. I haven’t got a use for it at the moment, but it looks like a nifty editor and I’d definitely like to have a play with it.