Wiki Notes and Stuff
The code running the wikipedia
http://wikipedia.sourceforge.net/
A wiki Travel guide:
http://www.wikitravel.org/en/article/Main_Page
An attempt at creating free textbooks with a wiki.
http://wikibooks.org/wiki/Wikibooks
A nice table that compares various wiki software:
http://moinmoin.wikiwikiweb.de/WikiEngineComparison
I like the look and philosphy of this wiki (PHP based):
http://www.pmichaud.com/wiki/PmWiki/PmWikiPhilosophy