(Quoted from http://www.bochs.com/)

The program bochs is a highly portable open source x86 PC emulator
written in C++, and runs on most popular platforms.  It includes
emulation of the Intel x86 CPU, common IO devices, and a custom BIOS.
Currently, bochs can be compiled to emulate a 386, 486 or Pentium CPU.
Bochs is capable of running most Operating Systems inside the emulation
including Linux, Windows'95, DOS, and recently Windows NT 4.

Source code for bochs is available by anonymous FTP under the LGPL.