musl
Alternative libc implementation, focused on correctness + size + static linking. Rich Felker, 2011. Standard in Alpine Linux, OpenWRT, many embedded systems.
~600 KB vs glibc ~10 MB. Modular, MIT-licensed (vs glibc's LGPL). Better with static linking (glibc discourages it). Distinct from glibc: stricter POSIX interpretation ⇒ sometimes breaks Python wheels, Node.js binaries. Pre-built musl-Linux toolchains (musl-gcc) popular for cross-compilation. Alpine + Docker made musl mainstream.