Europeana
Europeana is the main entry point to Europe's digitised cultural heritage. It gathers descriptions of paintings, books, maps, manuscripts, photographs, and audiovisual and 3D material from thousands of museums, libraries, galleries, and archives, and presents them through a single multilingual catalogue.
You can work with the data in several ways. The Search API returns metadata records and media, the Record API gives detailed information about individual items, and there is IIIF support for viewing high-resolution images and documents (IIIF is a shared standard for serving and displaying images across institutions). A SPARQL endpoint, which lets you query linked data using the SPARQL query language, connects Europeana to external sources such as VIAF, Iconclass, GeoNames, Wikidata, and DBpedia, and harvesting methods let you download metadata at scale.
For RAG, the metadata is the immediately usable layer: rich, structured, and released under CC0, so you can reuse it freely. The underlying item content varies in licence from item to item, so check the terms on anything you plan to use beyond the metadata itself.
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.
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.
Smithsonian Open Access
Millions of digital items from the Smithsonian's museums, research centres, and archives, released under CC0. Includes images, 3D models, and research datasets you can reuse freely, including commercially.