Andrew W. Mellon Foundation — Website: http://www.mellon.org/

Note: The source text explicitly lists 'official website http://www.mellon.org/' in the Wikidata entry for the Andrew W. Mellon Foundation. The claim states the website is 'http://www.mellon.org/' which matches exactly. Per the URL format guidance, differences in protocol (http vs https) and www prefix are not contradictions, so this is a clear confirmation.