We do not proxy live APIs: relaying adds latency and a point of failure and adds nothing. These answer directly, without a key or registration.
Terms change; each entry links to the current ones. We have no affiliation with any of them.

Weather forecasts and history
  Open-Meteo
  example: https://api.open-meteo.com/v1/forecast?latitude=52.52&longitude=13.41&current=temperature_2m
  terms: No key for non-commercial use, fair-use limits; data CC BY 4.0. https://open-meteo.com/en/terms

Air quality
  Open-Meteo Air Quality
  example: https://air-quality-api.open-meteo.com/v1/air-quality?latitude=52.52&longitude=13.41&current=pm2_5
  terms: No key for non-commercial use; CC BY 4.0. https://open-meteo.com/en/terms

DNS resolution (DNS over HTTPS)
  Cloudflare 1.1.1.1
  example: https://cloudflare-dns.com/dns-query?name=example.com&type=A (header accept: application/dns-json)
  terms: No key. Privacy policy: https://developers.cloudflare.com/1.1.1.1/privacy/public-dns-resolver/

DNS resolution (DNS over HTTPS, JSON by URL only)
  Google Public DNS
  example: https://dns.google/resolve?name=example.com&type=A
  terms: No key. https://developers.google.com/speed/public-dns/terms

Geocoding (address to coordinates)
  Nominatim (OpenStreetMap)
  example: https://nominatim.openstreetmap.org/search?q=Berlin&format=json
  terms: No key; strict usage policy: max 1 request/second, identifying User-Agent, no bulk use. https://operations.osmfoundation.org/policies/nominatim/

Scholarly reference lookup (DOI metadata)
  Crossref REST API
  example: https://api.crossref.org/works/10.1038/nature12373
  terms: No key; metadata mostly CC0; adding mailto= in the query joins the 'polite' pool. https://www.crossref.org/documentation/retrieve-metadata/rest-api/

Scholarly works, authors, institutions
  OpenAlex
  example: https://api.openalex.org/works/doi:10.1038/nature12373
  terms: No key for basic use; data CC0. https://docs.openalex.org/

Preprints
  arXiv API
  example: https://export.arxiv.org/api/query?search_query=all:electron&max_results=1
  terms: No key; max one request every 3 seconds. https://info.arxiv.org/help/api/tou.html

Registration data for domains and IPs
  RDAP via rdap.org
  example: https://rdap.org/domain/example.com
  terms: No key; redirects to the authoritative registry. https://about.rdap.org/

EU VAT number registration check
  VIES (European Commission)
  example: https://ec.europa.eu/taxation_customs/vies/rest-api/ms/DE/vat/123456789
  terms: No key; operated by the European Commission; subject to member-state availability. https://ec.europa.eu/taxation_customs/vies/

Wikipedia / Wikidata entities
  Wikimedia REST API
  example: https://en.wikipedia.org/api/rest_v1/page/summary/Earth
  terms: No key; identifying User-Agent required; content CC BY-SA, Wikidata CC0. https://api.wikimedia.org/wiki/Terms_of_use

# type: reference-data