Correct me if I'm wrong, but isn't Windows now the only viable operating system that isn't some form of -nix?
OSX has a server platform that is not bad. But that is BSD in the backend. The issue is that nobody really uses BSD except the really BIG places because BSD does networking better and has some more security stuff (not an expert and I anticipate a flame war). Redhat and Debian are widely available easy to use and have a ton of plugins and addons both free and paid. I'm running wikis on Ubuntu for the departments, for example, all on Ubuntu with a wiki-plugin from the Ubuntu repositories. The problem with OSX's server implementation is that is only runs on Apple hardware, is crazy expensive and is a locked down architecture where you need to buy apple approved software (unless that has changed in the last five years). I've only seen three servers in the wild; two were in the same site. here is the web server survey for some more info. I forgot about SUN; I guess Oracle still has some market share as well.
I would never put OsX on a server. That whole having configuration files in /etc/, but silently ignoring changes to some of them because they expect you to use some point and drool thing instead of editing them thing drives me nuts. I do believe someone would die if I ever had to lug a monitor, keyboard and mouse down to the colo facility in the middle of the night because how dare I try to bypass the shiny buttons Apple's godlike designers designed for me. It is my least favorite member of the unix family, and I have used Aix.
Yes. RiscOS is still around, but I don't think anyone uses it outside of embedded systems except as a toy, and Linux is probably more common in embedded systems now. Amiga tries to come back from the dead every once in a while, but it never happens.