Virtualization is one of those buzzwords that is being freely floated around the industry. Virtualization allows companies to cut cost by maximizing use of existing hardware resources.
The Xen hypervisor, the powerful open source industry standard for virtualization, offers a powerful, efficient, and secure feature set for virtualization of x86, x86_64, IA64, ARM, and other CPU architectures. It supports a wide range of guest operating systems including Windows, Linux, Solaris, and various versions of the BSD operating systems.
PearPC is an architecture-independent PowerPC platform emulator capable of running many PowerPC operating systems, including Mac OS X, Darwin and Linux.
KVM (for Kernel-based Virtual Machine) is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V). 
OVPsim is a multiprocessor platform emulator that uses dynamic binary translation technology to achieve high simulation speeds. It has public APIs allowing users to create their own processor, peripheral and platform models. 