Mac-on-Linux is an open source virtual machine program for running Mac OS on PowerPC computers running Linux. It can also be used to run another instance of another PowerPC-based operating system 
Linux-VServer provides virtualization for GNU/Linux systems. This is accomplished by kernel level isolation. It allows to run multiple virtual units at once. Those units are sufficiently isolated to guarantee the required security, but utilize available resources efficiently, as they run on the same kernel.
This site contains information relating to the use and development of virtual servers based on Linux-VServer. This particular virtual server model is implemented through a combination of “se
JPC is an x86 emulator written in pure Java. It can run on any platform that supports the Java Virtual Machine. It creates a virtual PC compatible machine that can run MS-DOS and other x86 operating systems. Programs inside JPC can run up to 20% of the native processor speed. 
The Hercules emulator is a computer program which allows software designed for IBM mainframe computers (System/370, System/390 and zSeries) to be run on other types of computer hardware: notably on low-cost personal computers. Although there are other mainframe emulators which perform a similar function, Hercules is significant in that it enables private individuals to run mainframe computer software on their own personal computers.