Ymacs is an Emacs-like editor that works in your browser. Which applies, at this state of affairs, only if your browser is Firefox. It looks pretty good with other browsers, but it is unable to catch all the required key bindings at the moment.
If you are an Emacs user, then you should feel like home with Ymacs. By default Ymacs uses Emacs idioms and key bindings. It should be easy to customize it, however, to mimic other editors.
License: BSD License