I think you can install mediawiki to your machine, and that would be exactly what you want.
I am currently using TikiWin. It is a wiki you can run on your desktop, and its tags are quite close to MediaWiki's. The major difference I guess is that It is small and easy to install.
If you can read Japanese (I know Mark can), and using a Windows computer, you may want to try it.
You can get it here: http://todo.org/download/tikiwin/
And I am relatively sure that if you look for it there must be something similar. I would like to know other desktop wikis.
happy editing!
Tomos