I would be interested in any search engine or directory that find wiki sites and only wiki sites.

Here are several search engines for wiki sites:
http://www.google.com/search?num=100&hl=en&lr=&newwindow=1&safe=off&q=%22wiki+search+engine%22&btnG=Search

Or if you are using Firefox (http://www.mozilla.com/firefox/upgrade-2.html) there is a whole bunch of plugins for Wiki search.
Link (http://mycroft.mozdev.org/download.html?name=wiki&submitform=Search)