One of the other free UnixOperatingSystems.
FreeBsd (usually written as FreeBSD), along with NetBsd and OpenBsd are descended from the UNIX/"Unixy" systems developed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley (CalBerkeley) in conjunction with AT&T in the 1970s and 1980s. In the early 1990s, a lawsuit was settled that ended with the last of the "proprietary" source code being removed, and it's no longer "owned" by any company.
It used to be a server-centric OperatingSystem rather than a personal computer OS. But since its version 4, it has taken up the task of conquering Linux. DragonFlyBsd is a descendant/branch of the version 4 of FreeBsd. According to a poll that was conducted by the BsdCertificationGroup in 2005, FreeBSD is the most popular BsdOs distribution.
FreeBsd invented the PortsTree which is now used in a similar way by other (even LinuxOs) distributions.
They also call them packages since Ports can also mean different platform ports (NetBsd might even call their packages something else).
External links:
FreeBSD is this operating system, which comes from BSD, which means that it is RealUnix. I like RealUnix, so therefore I like FreeBSD.