Wikimedia Commons
Wikimedia Commons is the shared media library behind Wikipedia and its sister projects. It holds more than 100 million files: photographs, illustrations, diagrams, audio recordings, and video. Content on Commons may be used by anyone, anywhere, and for any purpose, subject to the specific licence on each file.
Every file carries structured metadata through Structured Data on Commons, which describes what a file shows, who created it, and how it is licensed in a machine-readable form. That makes it possible to query and filter the collection programmatically rather than scraping pages.
Licences vary by file: some are released under CC0 or are in the public domain, while others use CC BY-SA, a share-alike licence meaning any derivative you distribute must carry the same terms. Because the file content and its metadata can sit under different licences, check each file before you build on it, especially for commercial use.
Related sources
Common European Data Space for Cultural Heritage
The umbrella project coordinating cultural heritage data across Europe. Led by the Europeana Initiative, it helps cultural institutions and EU Member States share their collections through a common, shared infrastructure.
Digital Public Library of America (DPLA)
An aggregator that brings together content from America's museums, libraries, and archives. Its API offers metadata on individual items and on collections, and the whole repository is available to download as zipped JSON files.
Europeana
A platform for discovering cultural heritage collections across Europe, with multilingual access to over 60 million digitised items from around 4,000 institutions, including books, paintings, maps, manuscripts, and audiovisual and 3D media.
HathiTrust Digital Library
A partnership of academic and research institutions offering millions of digitised titles. Its Research Center provides computational access to the full corpus, including in-copyright works, under non-consumptive research terms.