musl libc is a free, lightweight libc implementation intended for use on embedded systems. musl weighs in at 462kB for a full .a library (compared to glibc's 2MB), making musl an attractive option. (See the libc comparison data here)
If you need a complete libc implementation for your system, this is an excellent place to start.
I also frequently refer to it when I need to a reference implementation or a complete port a new libc function.
musl is usually more accessible for function porting than Apple's open source libc, which relies on FreeBSD implementations and platform-specific assembly code.
musl libc does not always have the best implementations (e.g.
memmove uses modulo operations), but most functions are implemented well.