Classic Mac OS seems to be the system from that era that's no longer available. There's still Amiga systems on the market. RISC OS is available for Raspberry Pi. DOS lives on in the form of FreeDOS. And now there are two OS/2 distributions (eComStation and ArcaOS).
You can't forget BeOS, currently alive in the form of the open-source reimplementation Haiku. In spite of their perpetual Alpha status, I'm still continuously amazed that it's even half as good as it is.