WYSIWYG editor

A WYSIWYG editor uses clever Javascript to replace the textarea input field, on the fly, with an interactive editor interface that allows users to create and format their posts the same way you would in a typical word processor. No scary HTML required! Even HTML savvy types will find that good WYSIWYG editors offer a fast and convenient alternative to formatting HTML by hand.

The top two editors are TinyMCE and CKEditor
http://www.tinymce.com/tryit/full.php
http://ckeditor.com/demo#full

From Google trend CKEditor is surpassing TinyMCE. I am using TinyMCE now, but its documentation is too simple to say it is documented.

There are a couple of other editors:
fckeditor (CKeditor's parent, and performance is not good)
kindeditor
xheditor
aloha-editor

There is a blog http://www.1stwebdesigner.com/design/10-best-wysiwyg-text-and-html-editors-for-your-next-project/ lists out 10 WYSIWYG editors, and the standout two are still TinyMCE and CKEditor.