MVS is an IBM operating system for the System/360 and System/370 computers. It is popular with hobbyists as full distributions are available and it runs very well under the Hercules emulator. There is a "turnkey" distribution available.
http://www.bsp-gmbh.com/turnkey/ ... Hercules runs MVS 3.8j (the last freely available release) very well. You can even assemble the whole system from source if you want!