Alpine Linux
Ultra-lightweight Linux distro built on musl + BusyBox + OpenRC. ~5 MB base. The de-facto standard for Docker base images.
Natanael Copa founded it in 2005. Apk package manager. PaX/grsec kernel hardening (formerly). Pitch: minimal attack surface, fast images. Catch: musl ↔ glibc incompatibility sometimes breaks Python wheels, Node.js binaries ⇒ distroless + Debian slim are modern Docker alternatives. Sizes: alpine image is 5 MB, ubuntu is 70 MB, debian-slim 80 MB.