http://moinmo.in/
Wiki written in python as used by Ubuntu, FreeBSD, Debian and others. Uses flat file storage by default. Lovely.
http://www.tiddlywiki.com/
Local HTML only wiki, all in one file. Useful for personal notes and so on. Not sure how it saves.
http://www.jigluhood.com/
JiglueHood allows you to create a free "group space" including a wiki, mailing list and possibly more. It uses the jiglu auto-tagging engine to link stuff together.
http://moulinwiki.org/
Moulin allows you to download different languages versions of Wikipedia on a single CD. It's aimed at developing countries where internet access is less common.
http://www.wikia.com/wiki/Wikia
Wikia allows you to create free (I think) MediaWiki installs. They currently host over 4'700 apparently.
http://wikkawiki.org/HomePage
Wikka is a very simple PHP/ MySQL wiki. It has very basic functionality, without the advanced features of something like MediaWiki, but it's extremely easy to use.
http://jottit.com/
Create web sites just by entering some text at jottit.com. A handy little app, with a beautifully simple interface!
