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

I've been building open source toolkits for free APIs for the past two weeks, and I keep finding gems that almost nobody talks about. My top 3 surprises: 1. OpenAlex — 250 million academic papers, completely free, no API key. I had no idea this existed until a month ago. It's like Google Scholar but with a proper API. 2. ClinicalTrials.gov API — 500K+ clinical trials, searchable by drug, sponsor, condition. Want to know what Pfizer is testing right now? One API call. 3. Unpaywall — Give it a DOI, it tells you if a free legal PDF exists. Saved me from so many paywalls. I built Python toolkits for all of these: github.com/spinov001-art Your turn: What free API surprised you the most? Could be anything — data, utilities, fun stuff. I'm always looking for new ones to build toolkits around. Bonus: if you share one I haven't heard of, I'll build a Python toolkit for it and credit you in the README. 🤝