Digital Public Library of America brings together metadata from libraries, archives, and museums in the US, and makes it freely available via their web portal as well as an API. DPLA's portal and API don't provide the items themselves from contributing institutions, but they provide links to make it easy to find things. The kinds of things DPLA holds metadata for include images of works held in museums, photographs from various photographic collections, texts, sounds, and moving images.DPLA has a great API with good documentation - a rare thing in this world. Further documentation on their API can be found on their search fields and examples of queries. Metadata schema information here.