glibc GNU C Library
GNU's libc implementation — de-facto standard in nearly all Linux distros. Big, complex, backward-compatible far into the past.
Roland McGrath started it in the 80s; Ulrich Drepper was the main maintainer 1995–2012 (controversial figure). Symbol versioning lets glibc 2.36 still run binaries linked against glibc 2.0. Size: ~10 MB. Competitor musl is ~600 KB. glibc-related bugs — Ghost (CVE-2015-0235), gethostbyname flaws — have been major security events. Building from source takes 30+ min.