BSD
From TWUUG
FreeBSD is an advanced operating system for x86 compatible (including Pentium® and Athlonâ¢), amd64 compatible (including Opteronâ¢, Athlon 64, and EM64T), Alpha/AXP, IA-64, PC-98 and UltraSPARC® architectures. It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley. -Copied from front page of FreeBSD.org
FreeBSD is by and large the most popular BSD distribution.
BSD distribution with focus on security. Their webpage claims only one remote hole in the default install in more than 8 years. Produced by the same organization that produces OpenSSH, the leading standard for remotely accesing *nix servers.
Very portable BSD distribution.
