Hercules
From Computer History Wiki
From the Hercules site:
Hercules is an open source software implementation of the mainframe System/370 and ESA/390 architectures, in addition to the new 64-bit z/Architecture.
That's right! Get a System/360, System/370 or System/390 emulator on your home PC!
Furthermore, there's a variety of public domain software from the 1960s and 1970s which will run under Hercules: