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.

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

WWW: http://wiki.qemu.org/Main_Page

In preparation for updating emulators/qemu to 4.2.X branch