FreeBsd

FreeBSD ( or the other BSDs for that matter) have been around a lot longer than linux. Although I like both linux and the BSD family, I tend to prefer doing work with FreeBSD. FreeBSD had some revolutionary ideas long before certain linux distributions decided to copy them. For instance, the package management tool in FreeBSD automatically does dependency checking and downloads any necessary packages when you install some software. Heck, it downloads the software as source and even compiles it on your machine. BSD has had this a few decades before Gentoo linux =P Some links:

1. Main Site: http://www.freebsd.org/
2. Handbook: http://www.freebsd.org/handbook/

Did you know that the stability of the BSD family of operating systems claims most of the top 10 spots on the netcraft uptime survey? Check it out!