QEMU is a FAST! processor emulator using dynamic translation to achieve
good emulation speed.
QEMU has two operating modes:

    * Full system emulation. In this mode, QEMU emulates a full system
(for example a PC), including a processor and various peripherials.
It can be used to launch different Operating Systems without rebooting
the PC or to debug system code.
    * User mode emulation (Linux host only). In this mode, QEMU can launch
Linux processes compiled for one CPU on another CPU. It can be used to
launch the Wine Windows API emulator or to ease cross-compilation and
cross-debugging. 

As QEMU requires no host kernel patches to run, it is very safe and easy to use.
(but kqemu is now also supported for the i386 on i386 and amd64 case)

See also the preconfigured system images on http://oszoo.org/
Many live cd isos also work.

WWW: http://fabrice.bellard.free.fr/qemu/