OPenn — Open Manuscript Data
Penn Libraries' bulk open-data site publishing high-resolution manuscript images and TEI/XML descriptions in the public domain, laid out as a plainly-navigable directory tree (openn.library.upenn.edu/Data/ returned HTTP 200). Not an API — there is no query interface — but it is genuine institution-operated machine-readable open data, and recorded as such rather than padded into an API entry.