Hiki -- http://hikiwiki.org/ compact, feature complete wiki usable on low end browsers (dillo). On better browsers some Ajax goodies are available.
The setup is easy, no dependencies except ruby, can use builtin webrick or external server. Needs to copy a few files and do some manual configuration before first startup.
The feature list is overseeable but fully sufficient. Includes good history, bbs and issue tracking plugins, RSS and web admin interface.
Very good hiki-mode.el for (X)Emacs freaks, lets you edit the wiki with the full comfort of a real editor. Usage of that mode could be slightly streamlined but well worth the effort if you enjoy (X)Emacs.
Has some support on http://eigenclass.org ( BrokenLink ) (triggers virus warning) which is a pretty good ruby oriented site running hiki btw.
Some Japanism's hidden inside code and docs, may need to fix content-type in a few places.
Data storage is plain text files in a flat direcotory with some metadata in an extra directory. So far no data disasters but if anything happens this is very reassuring:) Also the content is easy to index or process with external tools.
Hacking the code is fairly straightforward, no javaisms -- just plain old RubyLanguage. Writing plugins is very easy.