MediaWiki is the wiki tool that powers Wikipedia, so any wiki site based on MediaWiki has the same Wikipedia look and feel.
Not in the Top 100 in 2013
|www.mediawiki.org/||Free – Open Source||Download|
Comments from some of those who selected MediaWiki as one of their Top Tools
“Wiki love. And I’ve only been using it for my own project documentation. Haven’t even gotten into the collaborative aspect.” Cammy Bean
“most collaborative work in our organization is done in a wiki and mediawiki is the tool of choice” Kevin Gamble
“simple, free, easy” David Flanders
“the engine behind wikipedia and one of the best, if you want to implement your own wiki” Michael Prophet