SimICS is an instruction-set simulator and operating system emulator for a variety of architectures, including PowerPC, Sparc, x86, MIPS and ARM. It is a powerful tool for computer architecture research, program debugging, and performance analysis of software systems.
There used to be an older version available for free; unfortunately this has now disappeared. The commercial version's site is http://www.windriver.com/products/simics/index.html. University faculties can obtain a copy for free, see http://www.windriver.com/universities/.