Free Free PC Emulators

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.

DOSEMU stands for DOS Emulation, and allows you to run DOS and many DOS programs, including many DPMI applications such as DOOM and Windows 3.1, under Linux.

DOSBox emulates an Intel x86 PC, complete with sound, graphics, mouse, (modem)?, etc., necessary for running many old DOS games that simply cannot be run on modern PCs and operating systems, such as Microsoft Windows 2000, Windows XP, Linux and FreeBSD.

QEMU CPU Emulator
QEMU is an emulator for various CPUs. It works on Linux, Windows, FreeBSD and Mac OS X. Windows version is in an alpha stage. See the website for further details.

Wine is an Open Source implementation of the Windows API on top of X and Unix.

Xen virtual machine monitor
Xen is a virtual machine monitor for x86 that supports execution of multiple guest operating systems with unprecedented levels of performance and resource isolation. Xen is Open Source software, released under the terms of the GNU General Public License