What's the Most Underrated Free API You've Used?

I've been exploring free APIs for the past 2 weeks and documenting every useful one I find. Some of my favorites so far: arXiv API — Search 2M+ research papers, no key needed PyPI JSON API — Get metadata on any Python package instantly npm Registry API — Discover packages by keyword, zero auth Libraries.io — Track 8M+ packages across 40+ platforms Hacker News API — Real-time tech news via Firebase But I know there are MANY more hidden gems out there. What's a free API you've used that most developers don't know about? Bonus points if: No API key required Returns JSON Has a genuinely useful dataset I'm collecting these into a comprehensive list on GitHub and would love to add your suggestions (with credit!). Related articles if you want to see code examples: arXiv API tutorial npm Registry API tutorial GitHub API: Portfolio Analyzer