Taxonomy TinyMCE
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