I understand that an alternative to a large mediawiki document is to create a tree of small mediawiki pages. Still, I sometimes prefer to create a document as a single page; sometimes I want a modernist linear narrative rather than a postmodern nonlinear link-fest.
For small to medium-sized documents, I like to use SubEthaEdit because it's a good basic editor with a user-contributed mediawiki syntax coloring plugin.
For medium to large documents, where the topic folding, editing, and navigation offered by an outliner is very helpful, I like to use OmniOutliner Pro. I've modified Fletcher T. Penney's MarkDown XSLT OmniOutliner plugin to export from OmniOutliner Pro to mediawiki.
You can download my MediaWiki export plugin from my software area or directly from here.
You can also download the source Xcode project.