The WYSIWYG Report is a pretty good overview of real-time editors available for the web. The List is probably the most complete listing available.
Related bits: Bug 97284 (allow page to make arbitrary elements user-editable in browser), Bug 88049 (Support .selectionStart & friends for textareas), xopus (moz2ie.js), O’Reilly: Are Rich Clients Taking Off or Tanking?