A Free Collaborative Knowledge Base. The online encyclopedia Wikipedia is being supplemented by user edited structured data, available for free to anyone.
Denny Vrandečić, Markus Krötzsch
Communications of the ACM, vol. 57 (2014), pp. 78-85. pdf 
Unnoticed by most of its readers, Wikipedia is currently undergoing dramatic changes, as its sister project Wikidata introduces a new multilingual ‘Wikipedia for data’ to manage the factual information of the popular online encyclopedia. With Wikipedia’s data becoming cleaned and integrated in a single location, opportunities arise for many new applications.
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See Denny Vrandečić