qemu

qemu is an emulator. It is very fast.

With qemu you can run a linux program compiled for a different architecture. For example, you can run herkforth on x86 GNU/Linux.

qemu can also emulate whole opperating systems, though at the time of this writing PPC operating systems are in early development stages.

See their site for more details: http://fabrice.bellard.free.fr/qemu/