CPU Emulator

There are many open source CPU emulators, which can be used to run different OSes without reboot. That's cool. I have to find time to study them. To name a few:

QEMU CPU Emulator
QEMU has two operating modes:
Full system emulation. In this mode, QEMU emulates a full system (for example a PC), including a processor and various peripherials. It can be used to launch different Operating Systems without rebooting the PC or to debug system code.
User mode emulation (Linux host only). In this mode, QEMU can launch Linux processes compiled for one CPU on another CPU. It can be used to launch the Wine Windows API emulator or to ease cross-compilation and cross-debugging.

Bochs IA-32 Emulator Project
Bochs is a highly portable open source IA-32 (x86) PC emulator written in C++, that runs on most popular platforms. It includes emulation of the Intel x86 CPU, common I/O devices, and a custom BIOS. Currently, Bochs can be compiled to emulate a 386, 486, Pentium, Pentium Pro or AMD64 CPU, including optional MMX, SSE, SSE2 and 3DNow instructions.Bochs is capable of running most Operating Systems inside the emulation including Linux, Windows® 95, DOS, and Windows® NT 4. Bochs was written by Kevin Lawton and is currently maintained by this project. Bochs can be compiled and used in a variety of modes, some which are still in development. The 'typical' use of bochs is to provide complete x86 PC emulation, including the x86 processor, hardware devices, and memory. This allows you to run OS's and software within the emulator on your workstation, much like you have a machine inside of a machine. For instance, let's say your workstation is a Unix/X11 workstation, but you want to run Win'95 applications. Bochs will allow you to run Win 95 and associated software on your Unix/X11 workstation, displaying a window on your workstation, simulating a monitor on a PC.

SkyEye Project - ARM Emulator
Now that the following hardwares can be simulated by SkyEye:
CPU CORE: ARM7TDMI, ARM720T,StrongARM, XScale
APPLICATION CPU: Atmel AT91/X40,Cirrus CIRRUS LOGIC EP7312,Intel SA1100/SA1110, Intel PXA 25x
MEMORY: RAM, ROM, Flash
Peripheral: Timer, UART, ne2k network chip,etc.
Now that the following Operating Systems and syetem softwares can run in SkyEye
uC/OSII-2.x with network support
uClinux based on Linux2.4.x with network support
ARM Linux 2.4.x
lwIP on uC/OSII
applications on uC/OSII, uClinux,ARM Linux

PearPC - PowerPC Architecture Emulator
PearPC is an architecture-independent PowerPC platform emulator capable of running most PowerPC operating systems.