SimNow is an emulator/debugger from AMD originally targeted at their 64-bit processors, but also applicable to Intel processors. It is a very useful tool for debugging at the assembler level; particularly helpful when debugging boot loaders or the early stages of OS code. You can single-step through code and watch memory and registers changing. Although usable as an emulator, SimNow is a little slow for that purpose.
SimNow is available for free for both Windows and Linux. See http://developer.amd.com/tools/cpu-development/simnow-simulator/ .