The Comprehensive Knowledge Archive Network is the place to search for open knowledge resources as well as register your own. CKAN has a wiki-like interface that lets anyone add and correct packages. CKAN is developed and maintained by the Open Knowledge Foundation. Both the CKAN code and data are open: free for anyone to use and reuse.
Wikipedia is a user-driven collaborative encyclopaedia with, according to their website, nearly 78 million visitors monthly, and more than 91,000 active contributors working on more than 16,000,000 articles in more than 270 languages.