WikiEditor
From MediaWiki Beginners Wiki
WikiEditor is an extension that makes editing in MediaWiki much easier.
By default, MediaWiki comes with a poorly defined toolbar and not much else but a preview button to assist in editing. WikiEditor, on the other hand, enhances the editing to be much like editing on a slightly enhanced version of Windows Wordpad, as opposed to the stock editor, which frankly is very, very bare bones.
It also provides special character insertion, a quick cheatsheet for basic MediaWiki markup, and a few additional options to enhance previewing.