The Hercules System/370, ESA/390, and z/Architecture Emulator
hercules-390.org
hercules-390.org
https://news.ycombinator.com/item?id=6094482
Taking one of the MVS 3.8j "turnkey" distributions for a spin is a fun path down nostalgia lane, and if you can figure out how to license/acquire z/OS, z/Linux and z/VM, you can set up a pretty fun virtualized mainframe environment.
http://www-304.ibm.com/partnerworld/wps/servlet/ContentHandl...
They rapidly got sued by IBM, tried to counter-sue on antitrust / unfair competition grounds, and (amusingly, when you consider the suit) ended up selling to IBM instead.
In addition to patents and the standard "thou shalt not reverse engineer" licensing clause, IBM also contractually bind their customers to run z/OS on only their hardware, although they also maintain a first-party emulator for development and test situations which don't demand a real zSeries.
I don't think Hercules would do fantastically as a commercial fork anyway - it's a great tool for play and development, but using it also basically eliminates any of the reasons to buy a mainframe in the first place - you certainly don't get lock-step / verified execution, lots of IO channels, or hot-swappable core hardware by using an emulator.