While I can't hope to do justice to this question, FreeBSD is a fully functional unix system, with a kernel and userland wholly its own (licensed with the BSD license) as well as a well maintained ports tree that brings in most of the unix application universe (x windows, windows managers, databases, programming environments, etc) and can be used anywhere a unix system is desired.

That said, it is very popular in networking applications (Junos the OS for Juniper network switches is based on FreeBSD) and other embedded situations (Mac OS X has a forked version of FreeBSD as its unix layer. Most people won't characterize OS X as having FreeBSD lineage any more, but at its root, there were some parts that were brought over in the early days.) Sony PS3/4 also has some parts based on FreeBSD.

See http://en.wikipedia.org/wiki/List_of_products_based_on_FreeB... for a partial list of interesting applications based on FreeBSD.

It is especially appealing as a non-linux opensource project with a BSD (non GPL) license that is fairly well maintained. The Non-GPL license means that a company can use FreeBSD in a closed-source project.

Usually the FreeBSD core maintainers care more about correctness and principles more than pure performance, so in a straight performance shootout linux would probably still win, but FreeBSD is close enough that it doesn't matter for the types of applications it's usually used in.

There is probably a ton more specific things that people will call me out on, but http://www.freebsd.org/advocacy/ (the advocacy working group) would be a good place to start digging if you are interested.