Transport for Denver, US vs Transport for Norway

RTD Denver's original gtfs-developer-guide.shtml page is gone after a full site rebuild (now 404s); the transit agency's open-data offering now lives under open-records/open-spatial-information, split into static GTFS zip downloads (bus, light rail, commuter rail, plus the regional Bustang service) and three GTFS-Realtime protocol-buffer feeds (vehicle positions, trip updates, alerts). No API key is required, only acceptance of a redistribution license agreement.

Entur is Norway's national registry for public transport data, aggregating schedules and real-time updates from 60+ operators (21,000 daily departures) behind a GraphQL Journey Planner API, plus SIRI real-time and NeTEx static feeds. Fully open and free of charge, keyless for read access, though Entur asks every consumer to self-identify with an ET-Client-Name header and reserves the right to rate-limit unidentified traffic; the old developer.entur.org portal is being retired in favor of developer.entur.no.