Every day, millions of people find answers on Wikipedia to questions both trivial and serious. Jack Herrick found his business model there... in January 2005 he started wikiHow, a how-to guide built on the same open-source software as Wikipedia, which lets anyone write and edit entries in a collaborative system. To his surprise he found that many of the entries generated by Internet users — free — were more informative than those written by freelancers.New Web Sites Seeking Profit in Wiki Model - New York Times
Check-out some of the sites mentioned in this article:
wikiHow, Wiki, Wikia, ShopWiki, Wikitravel, Wetpaint, PBWiki.