There are a few other systems built on top of the main BSDs. PC BSD, which is a modified version of FreeBSD, is aimed more at the desktop. It features a graphical installer and a set of default packages targeted at a desktop user. M0n0wall is another FreeBSD-derived system designed for use in firewalls.
There are also a few commercial BSD-derived products available. Contrary to the belief prevalent in the Linux community, many of them do contribute code back to the parent projects, even though the license does not require them to. BSDI, the company behind BSD/OS, was a long-time financial supporter of the FreeBSD project.
All the BSDs have the cohesive feel that comes only from the kernel, userland, and documentation being written by the same people, but they choice of which to use is a matter of personal taste. On a low-volume server, the security and ease of use of OpenBSD can be attractive. On a laptop, the hardware support of FreeBSD can be more attractive. On anything else, NetBSD may be the only choice, and on other hardware, the ease of package management and the lightweight design might make it a better choice. Code is frequently shared between the systems, so a feature you like in one is likely to eventually make its way into the other two.