For Old time Mainframers - If you download Hercules you can get a real working version of IBM's MVS 3.8 mainframe OS running on your laptop - complete with TSO sessions etc etc. !!!! Now this OS went out around the late 1970's !!! but you can still run it if you want to.
That is really interesting. Maybe one day I will try that. I spent a couple of years in the development of MVS components - that was in the mid 60's. Can you imagine - that was nearly 50 years ago.