WYSIWYG HTML Editors are usually web based editors which provide rich text and html editing and can be emebedded into PHP applications. It can run on any browser platform. Most are them listed here are free while the popular ones are commercial.
Tinymce is a free WYSIWYG html editor written in javascript and is released under open source LGPL license. It converts textarea to rich text html editor and can be embedded into php scripts. A lightweight editor with powerful features for editing html content. It can replace textarea into a rich text editor. It also supports many browser platforms. It is free to download. A very simple WYSIWYG BBCode editor that can be plugged into any php script. It is a text area replacement and it inserts bbcode to selected textarea values. It is free to use for personal or commercial purposes. 
Free WYSIWYG textarea replacement with powerful features and works well in most popular browsers like firefox, IE and safari. Wyzz is a free lightweight WYSIWYG editor under GPL. It can be used in any CMS (content management) , webmail applications and more. Its not heavy and under 10k in size. Lightweight markup kit with BBCode style editor and parser. 
A lightweight and fast loading rich text html editor with WYSIWYG environment. It is fast loading and is released under GPL. A textarea markup editor that can insert html, bbcodes or even wiki code in your textarea. It is lightweight and free for download. BBCode generator and PHP HTML decoder script with a nice editor interface. TextAreaRich is a free javascript based text area replacement. It can be integrated to web applications of PHP or ASP scripts. Cross browser rich text html editor that run on Firefox, IE and other browsers. It is free under Creative commons. A very light weight, full featured, multi language, WYSIWYG HTML Editor in Browser written in Javascript, DHTML and PHP. It can be used wherever a Rich Text is needed. It is free for download A free open source webbased WYSIWYM (What You See Is What You Mean) XHTML editor for your web applications. A free to use edit in place editor for website content. It is free to download under GNU license. 
RTWedit is a very simple free GPL WYSIWYG html editor written in javascript and can be plugged into php forms easily. It runs on firefox, Internet explorer and other browsers. It also supports design mode. A very simple html editor written in javascript with extended functionality. It works on any platform including browsers and it is free. With few lines of code, it converts plain textarea to wysiwyg html editor. WYSIWYG HTML based pop-up editor written as a JavaScript library and is designed to be plugged into Web forms. It is a free open source software released under open source software under the GNU Lesser General Public License.
