This HN link( http://news.ycombinator.com/item?id=4798791 ) introduced me to the knowledge that OpenBSD is commonly installed on networking gear, instead of (for example) Cisco IOS. So you could say "freebsd on your desktop and/or server, and openbsd on your router." I confess that I don't know enough about NetBSD to offer an optimal use case.

oh yeah, openbsd has tons of great networking stuff. pf, carp, openssh all have originated from openbsd.

I use openbsd on the desktop. It's great. Just works, and stays out of the way.

NetBSD on your toaster is the canonical example.

I run NetBSD as a web server, but not for any good reason -- it's just personal preference. I ended up with NetBSD rather than one of the other variants because it's easy to set up as a Xen domU (instructions at http://wiki.prgmr.com/mediawiki/index.php/NetBSD_as_a_DomU if anyone's interested).