What Free API Surprised You The Most?

I've been building Python toolkits for free APIs for the past few months. Some of them blew my mind — I had no idea these existed. My top 3 surprises: 1. Crossref — Every DOI Ever Assigned I didn't know there was a free API that holds metadata for 150 million scholarly articles . No API key. No signup. Just send a GET request. import requests article = requests . get ( " https://api.crossref.org/works/10.1038/nature12373 " ). json ()[ " message " ] print ( f " { article [ ' title ' ][ 0 ] } — cited { article [ ' is-referenced-by-count ' ] } times " ) Every DOI you've ever seen? The metadata lives in Crossref. 2. USGS Earthquake API — Every Earthquake in Real Time Every earthquake on Earth, updated every minute. Free. No key. resp = requests . get ( " https://earthquake.usgs.gov/fdsnws/event/1/query " , params = { " format " : " geojson " , " minmagnitude " : 5 , " limit " : 3 }). json () for eq in resp [ ' features ' ]: print ( f " M { eq [ ' properties ' ][ ' mag ' ] } — { eq [ ' prop