FCKeditor, one of the most popular free WYSIWYG editors around now has a new version with more features & a lighter name: CKEditor.
Also, it is now an accessible script in compliance with the W3C WCAG & the US Section 508.
CKEditor offers a rich & powerful integration and interaction API. The editor is also totally plugin based & can be extended/modified in all senses to fit every need.
With a new user interface based on a fresh skin which is colorful like a chameleon, you can change its color to match your needs.