I've created a modified version of rte-light for my current project. Uses an icon-sprite instead of single images for the toolbar and some other stuff I needed. Might be a good starting point.

TinyMCE and the like all have the problem of a massive core. You can just start with a plain editor and select the few features you ned. Instead you always get a massive default selection and some optional plugins.

Also, instead of of multiple editor instances (when someone needs more than one editor on a page), what if we have only one toolbar? And this can edit any of editable html content.

Hi i share here the work we've done in our company used in production environement for 2 years now.
we are waiting for buttons to be ready to use them as toolbars buttons and then rewrite the plugin to use ui-factory.
At this time it support possibility to plug your own function for some command and the possibility to put some user defined style in a selector ( apply style selector ).
There's no other documentation than the code itself if you want to have a look at it for inspiration or don't know... just to share our work.
here's the demo link:
http://dl.dropbox.com/u/1151318/jqueryRte/demo.html

I found another editor recently. The editor has the command plugin pattern support Yanick proposed. http://jhtmlarea.codeplex.com/

I've added my own plugin to add an 'image gallery'. When I click on my custom command it opens another page in a window where the user can pick one or more images to insert. I prefer to have users navigate to virtual images that can be inserted in the document instead of exposing a dialog that navigates to physical files on the server. I let users upload an image to a database or the cloud without exposing the storage mechanism to them. I think it may be beyond the scope for a base jQuery UI plugin to support navigating to images in a specific location. Support for entering an image url may be sufficient in the base plugin.

Hey sorry for the bad links earlier. Since the last post I've moved everything to a new host.
You can see the same demos here:
http://dev.ghinda.net/jquery_ui_rte/

and download
http://dev.ghinda.net/jquery_ui_rte.zip

This is more a concept than an actual usable rte, especialy theme integration, but I'll probably start some serious work on it in the following weeks, most probably using html5 contenteditable.

I tried http://jhtmlarea.codeplex.com/ and liked it's simplicity, so I implemented a widget factory enable richtext editor using what I proposed last October 2009, using jhtmlarea as a tutorial work. As you can see, it's fully extensible and is not embedded with bells and whistles. I just made this in a few hours (including the demo page) so it's really an early plugin, but I really think it's a good start.

The demo is here : http://mind2soft.com/labs/jquery/richtext/demo/#demo

Comments welcome (email provided in the "About" tab)

As lovely as much of the suggested editors here are I would really prefer to see something that does not use a separate iframe. iframes have one major problem: they can never use the same CSS as the site itself, so the text will always look different in an iframe based editor than it will look on a rendered page. This can be worked around by requiring people to write custom CSS for just the editor iframe, but that is a fragile and unrealistic approach.

I have seen one (non-open) editor take this approach and the result was beautiful, so it is possible.

Having an iframe-based editor built on top of an in-page editor would be pretty slick. If someone were to actually build an RTE from scratch, I think that would be an awesome goal.

A few free RTE
http://sixrevisions.com/user-interface/rich-text-editors-for-2010-and-beyond/

I still believe that this widget should not have a toolbar; but should rather have a rock solid API that can be controlled through external widgets (i.e. JQuery UI Toolbar). This would also allow "floating" toolbars, and such. Moreover, it would keep the widget clean and fully expandable. However, because a toolbar is necessary in just about (if not all) projects with a rich text editor, I would suggest to have a separate widget for that, one that would interface with the rich text editor. Something like

var editor = $('#text').richtext();
$('#texttoolbar').richtextToolbar({editor: editor]);

// change editor after instantiation
$('#texttoolbar').richtextToolbar('editor', $('#othertext').richtext());

I strongly suggest that no clean-up be done on the client-side, there's no use downloading hundreds of lines of JS to clean up the markup, when this can be done at the server.

I agree with Yanick that the editor should be very simple (though it should have a rich API), and be extended via plug-ins. The toolbar should be a plug-in (utilising http://wiki.jqueryui.com/Toolbar), as you might want to use a different UI (or none [keyboard-only]) for interacting with the editor.

It's essential that a jQuery RTE is made easy to add custom plugins to such as your own options on the toolbar. Having a server image browser would be great, but as it's much more complicated and beyond a basic RTE, this RTE would need to be able to accommodate a custom plugin that could provide that type of functionality.

I just picked up my 1 yr old project and updated the Git repository. A demo is available at http://mind2soft.com/labs/jquery/richtext/demo/ . I haven't tested thoroughly on every browsers yet, but it should be fairly compatible with the major ones for now. At best, all features should degrade gracefully. All comments welcome.

I saw (and consequently fell in love with) elRTE - http://elrte.org/demo