[eliseomartelli]

OpenBSD source code[1], because I think it represents good design. The code is as little as possible and easily understandable.

[1] https://cvsweb.openbsd.org

[yung_steezy]

I would always recommend the OpenBSD coreutils over the GNU coreutils as they are much cleaner and less hacky. To be fair many of the tricks being used in the GNU code make the code run much faster but they're not a great starting point for a casual code browse.