I haven't contributed to it yet, but one of my personal projects involves hacking NetBSD. Absolutely, legendarily good code base, so thoroughly documented you c

I haven't contributed to it yet, but one of my personal projects involves hacking NetBSD. Absolutely, legendarily good code base, so thoroughly documented you can actually begin kernel hacking just by reading the man pages. The code itself is really consistent and easy to read; it's pretty clear the NetBSD team enforces a consistent style as well as high standards for thorough documentation. NetBSD is how C code should be done. The other code base I really liked was QRes, ITA Software's airline reservation system written in Common Lisp. I contracted at Google a few years ago doing maintenance on this code base before it was shuttered for good. Aside from being just high-quality Lisp code, it had its own testing language that allowed new tests to be written in just a few lines or so -- tests that sent XML queries with generated data to the running system and checked for specific results in specific XPath paths. Because the system was Lisp, implementing and extending this testing language was relatively easy. Using it even more so. Truly the only system I was happy to write tests for, rather than seeing it as a necessary chore.