wikEd is a full-featured Wikipedia-integrated text editor that adds enhanced text processing functions to edit pages. Currently, it works not under Internet Explorer and Opera. wikEd is a complete rich-text pseudo-WYSIWYG editor and comes with many MediaWiki-specific features
It’s not WYSIWYG, obviously, but it has a lot of features that help you clean up and manage large swaths of wikitext.
You can use it on Wikipedia by enabling it under “My Preferences / Gadgets.”