Supporting *unix* beyond Linux and Mac

[markw-FJ05HQ0HCKaWd6l5hS35sQ at public.gmane.org]

As you probably know, I've been posting and asking for beta testers, I'm
developing a site that has a software component that must be run.

I currently support Mac, Linux, and Windows. I'm pretty sure that *any*
variant of unix could be supported, but there are a lot, netbsd, openbsd,
freebsd, etc.

Does anyone have a good argument about which to support if I can only
support one?