Editors
The currently supported editors are FCKeditor, jWYSIWYG, markItUp, NicEdit, openWYSIWYG, TinyMCE, Whizzywig, WYMeditor, and YUI editor.
NicEdit
MIT License.
openWYSIWYG
LGPL Open Source license. Looks nice. Supported browsers include: IE 5.5+ (Windows), Firefox 1.0+, Mozilla 1.3+ and Netscape 7+. I tried this first because of support for old browsers but had to go elsewhere because I use IMCE.
The main script file is 94KB which could be a big download for remote users but it is not as bad as TinyMCE and FCKeditor. The menu bar is empty until you go into Buttons and plugins to turn on buttons. Check the Cleanup and output settings to make sure the editor does not remove any formatting you need elsewhere.
The buttons have italics instead of em and bold instead of strong. :-(
markItUp
markItUp is a jQuery plugin and the Packed naked engine
is only 6.5KB. Licence: MIT/GPL. Supported browsers: IE7, Safari 3.1, Firefox 2, Firefox 3. IE6 and Opera 9+ as-is.
Whizzywig
MIT style License.
jWYSIWYG
GPL 2 license. This plugin is an inline content editor to allow editing rich HTML content on the fly. It's an alternative to WYMeditor with much less features. With a small file size less than 26Kb total and only 18Kb of code and 7Kb packed, the main concept is to keep it simple, not all users need font coloring or create tables, just the basic.
Considering that both jWYSIWYG and WYMeditor are under heavy development and growing in size, the comparison might change by the time you read this.
TinyMCE
LGPL. Their file and image management addons cost money. Use Wysiwyg image uploader or IMCE. TinyMCE competes with FCKEditor for the most buttons on the toolbar. The main script file is 175KB. There is a jQuery version that is just a few percent smaller at 163KB. The editor has 34 add ons each adding a button, an image, script files, and a CSS file. If you use all the add ons, you could blow up your browser because some browsers will download only a limited number of script files (or is that CSS files (It is an IE thing)).
There are many more buttons in this editor but they still have italics instead of em and bold instead of strong.
When you open existing content in TinyMCE for the first time, TinyMCE adds a weird collection of paragraphs and breaks. Content that Drupal displays as a series of paragraphs can become one giant paragraph with breaks inserted. Drupal displays a\nb as <p>a</p><p>b</p> but TinyMCE converts it to <p>a<br /><br />b</p>
YUI editor
. Standard and lite versions. BSD license. Nothing stands out against FCKeditor and similar editors except the BSD licence which lets you create and sell your own version.
The YUIeditor is buried in a 13.2MB download that expands to 83.7MB on disk.
WYMeditor
For the XML puritans. No fonts or anything. You define the CSS then your editors use your classes.
FCKeditor
Biggest baddest editor. Every feature you can think of plus the kitchen sink. Version 3 is called CKEditor so spam killers will not reject the FCK bit. Distributed under the GPL, LGPL and MPL open source licenses.
CKeditor
The CKeditor is FCKeditor version 3 and, by some reports, makes FCKeditor look like a snail with arthritis. You can use CKeditor through either the development release of this module, see issue http://drupal.org/node/462146, or the beta CKeditor module.
Multiple editors
You can install multiple editors and have a different editor for each Input format. Compare two editors site by side with one on Filtered HTML and the other on Full HTML. To change the editor for an input type, remove the exiting entry in Wysiwyg and select another editor.
Image Upload
IMCE connects to Wysiwyg through IMCE Wysiwyg bridge but IMCE Wysiwyg bridge works only with TinyMCE and FCKeditor. What is your experience with other editors and image upload?

FCK Editor replaced by CK Editor
Newest stable version CKEditor 3.0, released on 20 August 2009 still not working in Drupal. Work is being done for D6.
Old & New Media Integration
TinyMCE uses strong and em
I don't know if it's something from TinyMCE or from Wysiwyg API, but TinyMCE is using em and strong! You should edit this part case it doesn't apply:
There are many more buttons in this editor but they still have italics instead of em and bold instead of strong.
I was used to Fckeditor, but this time I tryed with all the editors available, I configure them the same way, and only with TinyMCE I manage to copy and paste a complicated word document just with control + V, and I got and XHTML valid document.
Exactly the right feedback
Thank you for your feedback. What release of Word?
Pasting documents is an important test. We might find people to test with other releases of Word plus OpenOffice and Abiword.
The editors should all have em and strong as standard and an option to switch off b, i.
petermoulding.com/web_architect
CSS file limit
IE is limited to using 31 stylesheets in total, regardless how they are included or added. If your editor does not appear to load in IE (it does, but isn't visible), try the 'Optimize CSS files' option on the Performance page. Seems to be no limit on script files.
FCKeditor and CKeditor are supported as two different editors by Wysiwyg module.
Unless there's an option to configure what tags to use for bold/italic/etc, we're stuck with what the editor feels is right. File an issue on the editor's tracker. Note that some editors have XHTML addons or plugins which override some of these things automatically.