Linux became the standard operation system for servers in the Internet over the last years. Most web server, gateways or mail server are built on this stable and well developed OS. But there is an
other giant in the backround working. FreeBSD has an long history and is known for it's stability and quality. It has some nice concepts like the ports collection which was adopted by the Gentoo distribution for the Linux world with there tool called portage. It has support for a lot of different cpu architectures and x86 hardware. And it's used as base for many commercial products for example JunOS.
If you want to have a look at FreeBSD you should start with the documentation and have some fun with experimenting.