I've played around with several WikiWikiWebs [1] (like TwikiK [2], which I find somewhat clunky, and UseModWiki [3], which I find okay) but none so far has impressed me more than TiddlyWiki [4], which has to be the easiest wiki to install—just one file and no web server required.
On the downside, it's just one file and I mean, everything is stored in that one file. It makes the editing of content pretty sweet and rather fast, but I'm guessing that as more content is added, the slower it gets since it all runs in the browser. An interesting twist might be to update content on the server using AJAX (Asychronous JavaScript And eXtensible Markup Language) [5], although I'll have to check to see if it's already been done [6].
[1] http://c2.com/cgi/wiki?WikiDesignPrinciples
[3] http://www.usemod.com/cgi-bin/wiki.pl
[4] http://www.tiddlywiki.com/
[5] http://en.wikipedia.org/wiki/Ajax_(programming)