This tutorial shows how you can install InnoTek's VirtualBox on a Ubuntu desktop. With VirtualBox you can create and run guest operating systems (“virtual machines”) such as Linux and Windows under a host operating system.
Read it at howtoforge
The VirtualBox binaries can be downloaded from here: http://www.virtualbox.org/wiki/Downloads. If the PUEL license is ok for you and there's a package for your distribution, you can install that package. For example, there is a VirtualBox .deb package for Ubuntu 6.10 (Edgy Eft), so if you use Ubuntu 6.10, you can use that package. I've also tested this package successfully on Ubuntu 7.04 (Feisty Fawn), so it seems you can use that package there, too.