Alpine Linux

Alpine trades the standard GNU userland (glibc, GNU coreutils) most distributions use for musl libc and BusyBox, producing container images and installs measured in single-digit megabytes rather than hundreds -- which is exactly why it became the default base image for a huge share of the Docker/container ecosystem. Its own package manager, apk, is fast and its repositories are extensive despite the distribution's minimal footprint, and its default install includes security hardening (PaX/grsecurity-influenced choices historically, and hardened defaults generally) beyond what a typical desktop distribution ships. It can run as a full desktop OS with Xfce installed, and remains popular for reviving genuinely old or resource-constrained hardware, but its primary real-world footprint today is as the base layer under an enormous number of container images.