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. Bochs can be compiled to emulate many different x86 CPUs, from early 386 to the most recent x86-64 Intel and AMD processors which may even not reached the market yet.
Bochs is capable of running most Operating Systems inside the emulation including Linux, DOS or Microsoft Windows. Bochs was originally 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.

Download is source code and must be compiled before use.

Requirements:
OS X 10.2+