This tutorial provides step-by-step instructions on how to install VMware Server 2 on an Ubuntu 8.04 desktop system. With VMware Server you can create and run guest operating systems (virtual machines) such as Linux, Windows, FreeBSD, etc. under a host operating system. This has the benefit that you can run multiple operating systems on the same hardware which saves a lot of money, and you can move virtual machines from one VMware Server to the next one (or to a system that has the VMware Player which is also free).

http://www.howtoforge.com/how-to-install-vmware-server-2-on-an-ubuntu-8.04-desktop