NetBSD was the first of the modern variants that is still actively developed. It started out life as a fork of 386BSD. The focus of NetBSD is portability which not only makes porting to new hardware easier (currently supporting over 60 different ports across many CPU architectures). Everything from a VAX, ARM & MIPS Windows CE based PDAs to a Sega Dreamcast and many other systems are supported and able to run the latest version of NetBSD. There is even a toaster which runs NetBSD The focus on portability also makes reusing components on other operating systems easy. For example the packaging system (forked from FreeBSD (which we will talk about next)) supports over 20 operating systems. This enables a consistent toolset to be used regardless of operating system.
Some of the highlights of NetBSD include ATF (Automated Test Framework), unprivileged builds and portable build infrastructure using build.sh.
Automated Test Framework, as the name suggests is used for automated tests of the source code to discover regression in the code base in an automated manner. Results can be found on the NetBSD release engineering page.
Unprivileged builds allow a user to not only build a copy of the operating systems without elevated privileges, but they can also build and install software from pkgsrc in a location they have write access to (by default, in a prefix under their home directory).
build.sh, the build framework, allows NetBSD to be built on any modern POSIX compliant operating system. Freeing the person to use a operating system of their choice to build releases.