WorldSixtySix (http://www.world66.com/ is a free online English-language wikitravel guide. In November 2003 it merged with CapitanCook, a similar project. The content of the site is licensed with the CreativeCommons AttributionShareAlike license. The software is written in PHP.
As of May 15th 2003 it had almost 1500 articles (conservative count).
See also WikiTravel, TravellersPoint, OpenGuides, WikiVoyage.