Without a doubt, the best free UNIX-like operating system available. NetBSD is known for it's portability, security, and stability. I would even go as far to say NetBSD is even better than OpenBSD. FreeBSD would be a close second to NetBSD. NetBSD is also superior to the UNIX ripoff operating system: "Linux".
Compared to to many distributions of Linux, such as "Slackware", NetBSD is easier to install, is more secure, and is more stable.