FreeBSD

FreeBSD is an advanced operating system for x86 compatible
(including Pentium® and Athlon™), amd64 compatible (including Opteron™,
Athlon™64, and EM64T), ARM, IA-64, PowerPC, PC-98 and UltraSPARC®
architectures.  It is derived from BSD, the version of UNIX® developed
at the University of California, Berkeley. It is developed and
maintained by a large team of individuals. Additional platforms are in various stages of development.

FreeBSD offers advanced networking, performance, security and compatibility features today which are still missing in other operating systems, even some of the best commercial ones.

FreeBSD makes an ideal Internet or Intranet
server. It provides robust network services under the heaviest loads
and uses memory efficiently to maintain good response times for
thousands of simultaneous user processes.