Microsoft Virtual PC, which is a quiet efficient tool but needs licences to work with. Today while browsing the web i came to get accross a free open source alternative: VirtualBox
I installed it, had a copy of Linux Ubuntu and created it new virtual guest OS, it took me about 1 hour to set everything up.
From VirtualBox website:
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See “About VirtualBox” for an introduction.
Presently, VirtualBox runs on Windows, Linux, Macintosh and OpenSolaris hosts and supports a large number of guest operating systems including but not limited to Windows (NT 4.0, 2000, XP, Server 2003, Vista), DOS/Windows 3.x, Linux (2.4 and 2.6), and OpenBSD.
VirtualBox is being actively developed with frequent releases and has an ever growing list of features, supported guest operating systems and platforms it runs on. VirtualBox is a community effort backed by a dedicated company: everyone is encouraged to contribute while Sun ensures the product always meets professional quality criteria.
That said, I seriously encourage any developer, who needs Virtual OS to run, to either to make a development platform or doing test.