VirtualBox: beginner and intermediate guide

VirtualBox is a virtualization product for the x86 architecture; started by Innotek, acquired by Sun and now maintained by Oracle.The software is licensed under the GNU General Public License (GPL).

Lifehacker has a couple of nice pages to introduce and make you proficient with VirtualBox:

• The Beginner's Guide
• How to run multiple operating systems side-by-side

VirtualBox can be downloaded from here. 

Several websites offer appliances for download: just google "virtualbox appliances"; the Sourceforge page looks interesting to me.

Android, FreeDOS, and LinuxMint on a OpenSUSE 11.3 desktop