Even before true CPU-supported "hypervisors," there was shim software like SoftICE that worked similarly to m1n1 in that you would run an OS underneath and then

Even before true CPU-supported "hypervisors," there was shim software like SoftICE that worked similarly to m1n1 in that you would run an OS underneath and then use a supervisor tool to trace and debug the OS under inspection. More recently, it's fairly common to use a hypervisor or simulator for kernel debugging in device driver development on Windows via Hyper-V. A lot of Linux driver development is done using qemu as well, although this is usually more targeted and isn't quite the same "put a thin shim over the OS running on the hardware" approach. The flexibility and I/O tracing framework in m1n1 are pretty uniquely powerful, though, since it was built for reverse engineering specifically.