Provision a fresh MongoDB database and get a scoped mongodb:// connection URL, backed by a per-database user.
MIT
MobilityData's open catalogue of public-transport feeds — ~4,500 GTFS schedule feeds and ~1,900…
Search the FAA handbooks, advisory circulars and Airman Certification Standards (ACS) used for pilot training, and read any page in full with its figures, charts and tables. Every result links back to the exact page so answers can be cited. Free, public, no API key.