What Free APIs Do You Actually Use in Production? (Not Just Tutorials)

I've been cataloging free APIs for months — I've found over 100 that have genuinely useful free tiers. But there's a huge gap between \"cool API I tried once\" and \"API I actually rely on in production.\"

My Production Stack (Free Tier Only)

Here are the ones I actually use regularly:

1. InternetDB (Shodan) — quick IP lookups, no key needed
2. crt.sh — subdomain discovery via certificate transparency
3. NVD — dependency vulnerability scanning in CI/CD
4. HIBP Passwords — checking user passwords against breach databases
5. GitHub API — repository automation (no token for public data)

What Surprised Me

• Most \"unlimited\" APIs aren't — they just don't document their rate limits until you hit them
• The best APIs are boring — NVD, RDAP, crt.sh. Not flashy, just reliable
• API keys ≠ paid — many require a free key just for rate tracking, not billing
• Government APIs are underrated — NVD, PubMed, arXiv. Funded by taxes, free forever

My Question to You

What free APIs are in YOUR production stack?

Not the ones from tutorials. The ones that would break something if they went down.

I'm specifically curious about:

• APIs you've used for 6+ months
• APIs that handle real traffic (not just dev/testing)
• APIs where the free tier is genuinely sufficient (not a trial)

Drop your stack below. I'll add the best ones to my curated list.