+1 for that. Plan 9's code, written in C, has a clean, minimalist aesthetic throughout that makes it dead easy to navigate, skim, or analyze in depth. Files, lines, and functions all tend to be short. Code within functions is often times linear with simple conditionals or loops; there's still complex logic to be found, but far less than other software; more than 4 levels of indentation is uncommon. Even the makefiles are simple and clean.
File, type, and function names are usually short and unambiguous. Variable names average 1-2 letters: r for Request, f for File, to and from for strings. Comments are used sparingly; when present, they give you the salient facts without unnecessary detail. They, too, are mostly short, but longer where more explanation is called for.
With all this brevity, you might expect the code would be cryptic or cramped, but it is extremely easy to follow, with a very "clean" and "natural" feel - easy on the eyes, with plenty of space. You can dive in at any random point and easily understand what is going on and why.
One may, of course, argue that the limited number of hardware platforms supported (half a dozen or so?) and operating systems (one) freed the authors from a huge amount of complexity and allowed them to keep their code simple. Could be. gcc's headers make my eyes bleed, but between POSIX and portability to every hardware and OS known to humanity, it's hard to fault them for it.
Overall, Plan 9's code is the cleanest, the easiest to understand, and possibly the most "beautiful" that I've seen.