Qemu runs on many different platforms, from ARM to x86 to Power PC to AMD 64. It allows you to run operating systems inside others.
The best thing about Qemu is that it allows you to create a 'PC-on-a-USB-stick'. That means that if you're unable to gain Admin rights to a computer, you can still run Linux on it! Unfortunately Qemu a little bit slower if run without hardware acceleration, which requires admin rights. Many other virtual machine software, such as VirtualBox and Xen, uses some of Qemu's drivers. It's a really useful emulator, and does things that many other virtualisation approaches can't do. Also, it's LGPL. Great application!