A TinyMCE Widget. Use the TinyMCE editor in a widget so you can insert it in any sidebar you like.
Insert a TinyMCE editor in any of your sidebars.
OTW TinyMCE Widget Options
If you need to further style the widget content generated by the OTW TinyMCE Widget here are your options:
This plugin comes Localization/Internationalization ready. It is following WP I18n standards.
Full version of the plugin