During the Lord of the Rings, they transitioned largely over to Linux for nearly the entire setup. That transition involved a lot of custom software, so it's probably still mostly Linux.
I notice that they didn't bother to say (or deliberately left out, at the orders of their corporate overlords) what operating system(s) this computing behemoth runs.
Ironic then that the final product is so heavily DRM'ed that you can't easily view it without breaking some laws if you're a free Linux user :)