SQLite
It's the most widely deployed database in the world.
SQLite is a public-domain database engine embedded in billions of devices - smartphones (iOS and Android), web browsers (Chrome, Firefox, Safari), operating systems (Windows, macOS), and applications (Skype, Dropbox, Adobe products). It's the most widely deployed database in the world.
SQLite is maintained by a team of three full-time developers and supported by corporate sponsors. It generates no licensing revenue (public domain) and employs minimal staff. Yet its removal would cascade through the software ecosystem, breaking millions of applications.
SQLite's keystone index is extreme: ~3 full-time developers and ~$0 direct revenue, but used by billions of devices. The impact-to-size ratio is astronomical - perhaps 10,000:1 or higher. SQLite demonstrates The Reliability Paradox: the most dangerous keystones work so well you've forgotten they exist.