VMware allows you to run 'virtual machines' inside a Linux host. It is not an emulator. It provides a virtual computer within the host which can boot whichever OS you decide to put on the filesystem image that is used as a harddrive. It will run DOS 6.22, Win 3.1, Win9x, WinNT/2000/XP/2003, Linux, Novell, and more. The only main requirement is a 400 MHz or better machine, along with lots of RAM (128M minimum, 256M recommended). License: Other/Proprietary License with Free Trial Changes:
Support for additional host and guest operating systems was added. Unity mode, allowing guest applications to integrate with the host desktop, was introduced. Accelerated 3D graphics are now available on Windows XP guests, supporting DirectX 9 up through Shader Model 2.0. VM record and replay was enhanced. The virtual hardware was updated for hot-plugging, to add LSI Logic SAS adapters, and for 3D graphics.

More...