There are many uses for the operating system FreeBSD, but we know FreeBSD especially for its reputation as a stable server operating system. Some of the servers in the 2006 /dev/null/nethack tournament used FreeBSD. (Compare FreeBSD with Debian, the OS behind nethack.alt.org.)
The FreeBSD Ports Collection is the easy way to install software onto FreeBSD, turning your machine into a nice web server or KDE box, for example. The other BSD operating systems forked their ports collections from FreeBSD. Two packages interesting to NetHack players are freebsd-games (for Hack, Larn, and Rogue) and nethack36.
A unique feature of FreeBSD is the jail. A jail has its own IP address and cannot see any files outside of the jail. Perhaps this convinced some tournament admins to use FreeBSD as a NetHack server, not another BSD or Unix. By locking the ssh or telnet server-daemon and NetHack in a jail, should there be a problem in security, the intruder is unable to attack anything outside of the jail.