libc
C's standard library — printf, malloc, fopen, strlen, etc. Sits on top of the kernel and is "the C API" to the operating system.
Implementations: glibc (GNU, dominates Linux desktop/server), musl (Alpine Linux, static-linking-friendly), bionic (Android), newlib (embedded), BSD libc (FreeBSD, OpenBSD, derived in macOS). glibc is notoriously backward-compatible — old binaries work in new glibc. musl is smaller, faster, more correct — but incompatible with some glibc extensions. Rust/Go programs can link directly against syscalls, avoiding libc entirely.