This plugin replaces a taxonomy term description textarea with the buildin TinyMCE WYSIWYG.
A category is a build in custom taxonomy. So yo can use this plugin for post categories and tags.

WordPress.org

Download it from the WordPress plugin directory

http://wordpress.org/extend/plugins/taxonomy-tinymce/

GitHub

You want to fork this?
https://github.com/jmslbam/Taxonomy-TinyMCE