Looking for zero-cost, production-friendly APIs? Here’s a curated list—organized by use case—with copy-paste examples, caveats, and links to official docs.

⚠️ Always review each API’s terms, rate limits, and attribution rules before shipping.

What “free” usually means

• Open & no key: Public endpoints, no signup (e.g., Open-Meteo, PokeAPI, Wikidata).
• Keyed free tier: Requires a token but offers a free monthly quota (e.g., NASA, Unsplash).
• Fair use/open data: Public but rate-limited (e.g., OpenStreetMap/Nominatim, Overpass).

Quick picks by category

• Weather: Open-Meteo, MET Norway, Open-Meteo Geocoding
• Maps & geodata: OSM Nominatim, Overpass API, Geoapify
• Space & science: NASA Open APIs
• Knowledge & content: Wikipedia REST, Wikidata, Open Library
• Code & dev: GitHub REST/GraphQL, Stack Exchange
• Fun & demo: PokeAPI, The Cat API, The Dog API, Deck of Cards, Numbers API
• Media: Unsplash API, Lorem Picsum
• Finance (public/EDU): Frankfurter, exchangerate.host
• News & civic data: GOV.UK APIs, GNews
• Testing/backends: JSONPlaceholder, ReqRes, HTTPBin

The list (what you get + why it’s useful)

Weather & Climate

1. Open-Meteo – Free forecasts & historical weather; no API key. Docs: https://open-meteo.com/en/docs
2. MET Norway (Locationforecast) – High-quality European forecasts; include attribution & a valid User-Agent. Docs: https://api.met.no/doc/
3. Open-Meteo Geocoding – Forward/reverse geocoding to pair with weather queries. Docs: https://open-meteo.com/en/docs/geocoding-api

Mapping & Geospatial

1. OpenStreetMap Nominatim – Geocoding from OSM; follow usage policy. Docs: https://nominatim.org/release-docs/latest/api/Overview/
2. Overpass API (OSM) – Query OSM features (cafés, EV chargers, paths). Docs: https://overpass-api.de/
3. Geoapify (free tier) – Geocoding, routing, isochrones. Docs: https://www.geoapify.com/developers

Space & Science

1. NASA APIs (APOD, NeoWs, EPIC, etc.) – Free key; perfect for content apps. Portal: https://api.nasa.gov/
2. USGS Earthquake API – Real-time quakes in GeoJSON. Docs: https://earthquake.usgs.gov/fdsnws/event/1/

Knowledge & Open Data

1. Wikipedia REST – Summaries, page content, and media metadata. Docs: https://www.mediawiki.org/wiki/REST_API
2. Wikidata – Query structured knowledge via SPARQL. Query Service: https://query.wikidata.org/
3. Open Library – Books, covers, authors. Docs: https://openlibrary.org/developers/api
4. OpenAlex – Scholarly works and authors. Docs: https://docs.openalex.org/

Developer & Community

1. GitHub REST/GraphQL – Repos, issues, actions. Docs: https://docs.github.com/en/rest
2. Stack Exchange – Q&A data across the network. Docs: https://api.stackexchange.com/

Media & Placeholders

1. Unsplash API – High-quality images; attribution requirements apply. Docs: https://unsplash.com/developers
2. Lorem Picsum – Simple placeholder images via URL params. Docs: https://picsum.photos/

Finance & Numbers

1. Frankfurter – Historical FX (ECB), no key. Docs: https://www.frankfurter.app/
2. exchangerate.host – Live/historical rates, no key. Docs: https://exchangerate.host/#/

News & Public Sector

1. GOV.UK APIs – Services & registers (UK). Index: https://www.api.gov.uk/
2. GNews (free tier) – News search JSON; simple, limits apply. Docs: https://gnews.io/

Fun, Learning & Demos

1. PokeAPI – Pokémon data; great for teaching pagination. Docs: https://pokeapi.co/
2. The Cat API – Random images & breeds. Docs: https://thecatapi.com/
3. The Dog API – Similar to Cat API, for dogs. Docs: https://thedogapi.com/
4. Deck of Cards API – Card shuffling/drawing. Docs: https://deckofcardsapi.com/
5. Numbers API – Facts about numbers, dates, math. Docs: http://numbersapi.com/
6. JSONPlaceholder – Fake REST endpoints for front-end devs. Docs: https://jsonplaceholder.typicode.com/
7. ReqRes – Mock user endpoints & auth flows. Docs: https://reqres.in/
8. HTTPBin – Inspect requests, headers, auth, etc. Docs: https://httpbin.org/

Copy-paste starters (curl, JS & Python)

1) Weather (Open-Meteo; no key)

curl "https://api.open-meteo.com/v1/forecast?latitude=21.03&longitude=105.85&hourly=temperature_2m"

// Browser/Node (fetch)
const url = 'https://api.open-meteo.com/v1/forecast?latitude=21.03&longitude=105.85&hourly=temperature_2m';
const data = await fetch(url).then(r => r.json());
console.log(data.hourly.temperature_2m.slice(0, 5));

2) Space image of the day (NASA APOD; free key)

curl "https://api.nasa.gov/planetary/apod?api_key=DEMO_KEY"

const url = 'https://api.nasa.gov/planetary/apod?api_key=DEMO_KEY';
const apod = await fetch(url).then(r => r.json());
document.body.innerHTML = `<h1>${apod.title}</h1><img src="${apod.url}" alt="">`;

3) Geocoding (Nominatim; add a real User-Agent)

curl -H "User-Agent: yourapp/1.0 (contact@example.com)" "https://nominatim.openstreetmap.org/search?q=Hanoi&format=json&limit=3"

4) Wikipedia summary (no key)

curl "https://en.wikipedia.org/api/rest_v1/page/summary/Blockchain"

5) GitHub repository info (token optional but recommended)

curl -H "Authorization: Bearer YOUR_GH_TOKEN" "https://api.github.com/repos/vercel/next.js"

6) Placeholder backend (JSONPlaceholder)

curl "https://jsonplaceholder.typicode.com/posts/1"

Patterns & best practices

1) Cache aggressively — Use ETag, Cache-Control, and client caches to respect free-tier rate limits.

2) Exponential backoff — Handle 429 Too Many Requests gracefully.

3) Identify your app — Fair-use APIs (Nominatim/Overpass) require a helpful User-Agent.

4) Secure your keys — Keep secrets server-side; proxy from your backend or edge functions.

5) Pagination helpers — Abstract page, per_page, next early to keep UIs smooth.

6) Respect licenses — Many open data/media APIs require attribution; add it in your footer.

7) Localize & fail gracefully — Provide empty states and fallback copy.

A minimal proxy (Node/Express) for hiding API keys

import express from "express";
import fetch from "node-fetch";

const app = express();

app.get("/api/apod", async (req, res) => {
  const r = await fetch(`https://api.nasa.gov/planetary/apod?api_key=${process.env.NASA_KEY}`);
  if (!r.ok) return res.status(r.status).json({ error: await r.text() });
  res.json(await r.json());
});

app.listen(3000, () => console.log("Proxy running on :3000"));

Testing checklist before launch

• Rate-limit & error handling (4xx/5xx, 429 backoff)
• Caching via ETag / Cache-Control
• Timeouts & circuit breakers
• Input validation & geofencing
• Keys in env vars (no client secrets)
• Attribution & licenses covered
• Observability: logs, request IDs, metrics

Where to discover more

• National/city open-data portals, academic consortia, OSS projects.
• Search: “{domain} open API documentation rate limits”.
• GitHub Discussions/Issues often surface real-world gotchas.