expand_less http://wikiohana.net/http://en.wikipedia.org/wiki/Wikihttp://open.wikiblogplanet.com/http://www.wikipatterns.com/
 
Personal wikis of note:
http://webseitz.fluxent.com/wiki/
http://webbrain.com/brainpage/brain/3D80058C-14D8-5361-0B61-A061F89BAF87
Some:http://zoho.com/