TiddleWiki is one of the most intesting wikis I've found. It consists of a single html file accessed through a browser window. It is fast and easy to start and modify. It does have its quirks, but overall my impression is positive. I have used it off and on for about six months as a preliminary to Wiki2008

Initial Impressions

There really is no downloading or installing. You almost have to see it to believe it and I will record a video of it in the near future.