TikiWiki is free wiki software that supports HeeHaw. It may soon obey the wikitext standard set by mediawiki and getwiki.
However, you will never figure that out by using the standard developer sites. See http://tikiwiki.sourceforge.net for incomprehensible and exclusionist discussion. See tikiwiki.org for something saner.
Alternate_wiki-implementations and talk:MediaWiki_modifications.