FreeBSD and OpenBSD are worth caring about, because both have their uses. Other variants are irrelevant. Just like how only a few Linux distros actually matter.
I wish Solaris would just curl up and die, but ZFS has it hanging on a bit longer. Oracle's Dtrace implementation on Linux is hopefully interesting and may lead to another nail in the coffin for SunOS. I strongly suspect that Oracle wouldn't mind seeing Solaris die off either.
And that is pretty much it for classic Unix systems. Between Microsoft and Linux there isn't much room left for it.
AIX and the like will still matter to some, but nobody wants to move to using those systems if they can help it. They are kinda like 'Blackbeard the pirate' during his heyday. Psychologically damaged relics of a bygone era.
Nowadays it's ridiculous to talk about portable software and not to automatically assume that you are including OS X or Windows into the discussion.