Emulators
There are at least three software-based System z mainframe emulators. FLEX-ES from Fundamental Software is a commercially offered option. The open source Hercules emulator supports Linux on System z (and can even run on Linux on System z itself), but IBM does not license other operating systems such as z/OS to run on Hercules. In 2010, IBM introduced the Rational Developer for System z Unit Test Feature which provides a restricted use execution environment that can run on X86 hardware. IBM's license terms limit use of the Unit Test Feature to certain application development tasks, not including final pre-production compiling or pre-production testing (such as stress testing). The Unit Test Feature includes z/OS (with common middleware) and is also compatible with Linux on System z.
Read more about this topic: Linux On System Z, Developer Resources