Create an Ubuntu Guest in VirtualBox
VirtualBox is a robust, easy to use, and freely available open source software developed by Sun Microsystems. VirtualBox allows you to run different Operating Systems in virtualization mode. For example, you can run Windows in Linux or vice versa using VirtualBox. I have put together a visual guide explaining how to create a new Ubuntu Virtual Machine in VirtualBox. Watch the video.
Also see how to install Guest Additions in an OS running in VirtualBox.