TinyMCE is an open source and platform independent WYSIWYG editor used by WordPress, Facebook, Joomla and other popular websites. It can convert HTML TEXTAREA fields or other HTML elements to editor instances. TinyMCE is a light weight and highly customizable editor.
CKEditor is an another open source rich text editor for web applications. It brings to the web common editing features found on desktop editing applications like Microsoft Word and OpenOffice. CKEditor was previously named as FCKEditor and is used by IBM, Oracle, Adobe and many other popular websites. You can find complete list here.
NicEdit is a very lightweight, cross platform editor licensed under MIT. The goal of NiceEdit is to be simple and fast as possible. NiceEdit can be easily integrated to your website form your website server as wel as from the niceEdit server remotely.
openWYSIWYG is a free, cross-browser rich text editor having all rich-text editing features. It has enhance table creation feature to help users construct HTML tables.
5- YI Rich Text Editor
YI is a rich text editor from Yahoo that replaces a standard HTML textarea. The YI rich text editor also have mobile devices support.
Tags: open source