There are both commercial and free Type II hypervisors for running Linux, DOS, or Windows on a Windows desktop computer.
Microsoft Virtual PC 2007 is free and will run on any computer running Windows XP or newer versions of Windows, and it supports Windows, Linux, and DOS guests without requiring hardware-assisted virtualization.
Windows XP Mode is a free hypervisor that installs on a Windows 7 host with a Windows XP guest preinstalled. It does not require hardware-assisted virtualization.
The free Windows Virtual PC requires both Windows 7 and hardware-assisted virtualization. Use the Hardware-Assisted Virtualization Detection Tool to test your computer before you download one of these solutions.
You have several choices for hypervisors for Mac OS X that will run versions of Windows and Linux.
Apple Boot Camp is not actually a hypervisor so much as a dual boot option that allows you to dual-boot between Mac OS X and Windows. This gives each OS full use of the hardware, but only one can be loaded at a time.