I've been building with free APIs for a few months now, and some of them genuinely shocked me with how much data they give away for free.
My top 3 surprises:
1. ClinicalTrials.gov - 500,000+ clinical trials, completely free, no API key. You can search by condition, drug, sponsor, phase. The data quality is insane because it's FDA-regulated.
2. USPTO PatentsView - 8 million+ US patents searchable via API. No key, no auth. You can search by inventor, company, technology class.
3. EPSS (Exploit Prediction) - FIRST.org gives you exploit probability scores for every CVE. Updated daily. Free. Most security teams don't even know this exists.
All three are government or nonprofit APIs, which is probably why they're free and unlimited.
I've been collecting these into open-source toolkits with ready-to-use Python code. Currently at 16 toolkits covering everything from weather to academic papers to vulnerability scanning.
What's an API that surprised you? Could be surprisingly good, surprisingly free, or surprisingly obscure.
Bonus points if most developers have never heard of it.
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