The Perl QA Wiki runs Usemod Wiki and was a pain in the ass to setup and configure.
The Unix-to-VMS HOWTO Wiki uses CGI::Kwiki, took me about five minutes, I like its Wiki syntax better, and the code isn't an embarassment.
I like its syntax so much it would be great if Slash offered it as a formatting option. Are ya listening, Pudge?
As a bonus, I can physically assault the CGI::Kwiki author should the need arise.