I’ve been using VMware Player for quite some time, and it’s quite good, but recently, I stumbled upon another free virtualization tool which also allows you to create new VM images (VMware Player only plays back VMware images created with a purchased version, such as VMware Workstation, or through a website such as EasyVMX: http://www.easyvmx.com/)
According to Wikipedia (http://en.wikipedia.org/wiki/VirtualBox):
VirtualBox is an x86 virtualization software package originally created by German software company Innotek and now being developed by Sun Microsystems as part of its Sun xVM virtualization platform. It is installed on an existing host operating system; within this application, additional operating systems, each known as a Guest OS, can be loaded and run, each with its own virtual environment. For example, Linux can be guest hosted on a single virtual machine running Microsoft Windows XP as the Host OS; or, XP and Windows Vista can run as guest OSes on a machine running OpenSolaris.
Supported host operating systems include Linux, Mac OS X, OS/2 Warp, Windows, and Solaris, while supported guest operating systems include FreeBSD, Linux, OpenBSD, OS/2 Warp, Windows and Solaris.[1]
According to a 2007 survey by DesktopLinux.com, VirtualBox is the third most popular software package for running Windows programs on Linux desktops.[2]
So far, so good, with the exception of a little glitch I experienced while installing FreeDOS. Ubuntu runs quite well, and quite fast, as far as I can tell.
A particularly cool element of VirtualBox is the “Seamless Desktop” mode, similar to what is found in Parallels or VMware Fusion on the Macintosh:
Considering the level of functionality given for free, which is quite similar to the functionality you’d have to pay for in VMware or Parallels, I’m tempted to stick with VirtualBox for the near future and put it through its paces.
If you’re curious about VirtualBox yourself, you can find a copy here:
http://www.virtualbox.org/wiki/Downloads
and more directly, here:
Sun xVM VirtualBox 1.6 Download
Update:
Here are some links to download pre-made VirtualBox images:
http://virtualbox.wordpress.com/
http://sourceforge.net/project/showfiles.php?group_id=213555
Williamo's Blog 
Here’s a great article about setting up networking in VirtualBox that was linking back to mine via the “Possibly related posts” on their blog:
http://opensourceexperiments.wordpress.com/2008/04/11/making-virtualbox-networking-work-with-suse-linux-as-guest-os-windows-host/
What about using Returnil? Do you think VirtualBox is better than using Returnil? What is your advice? I have been using Returnil for a while but it start to give some troubles – Crashes, Blue Screens, Gmail Save as draft crashes Firefox 3, none of these happens if I turn off Returnil Protection. Does Virtual box playsball with kaspersky Interenet Security 2009? I mean, having to turn off the antivirus – as suggested on the VirtualBox manual is not something I am sure I want to do. What about protection when surfing the Internet (Activex Controls, Java applets and Malaware in General – Such as Virus and spyware hiding in web pages) Does Virtualbox fully protects a PC with a simple reboot? Many thanks for your time.
I haven’t tried Returnil myself, so I can’t say how it compares to VirtualBox. I can say, however, that I’ve found VirtualBox to be very stable and that I haven’t had any problems using it with my preferred anti-virus program, AVG. I’d assume that it should work correctly with Kaspersky as well, given that it works just fine with AVG, but your mileage may vary.
I’m not quite sure what you mean by “Does Virtualbox fully protects a PC with a simple reboot”, though. You can surely isolate your virtual machine from your host environment, and I’m pretty sure that you can use snapshots to revert to a “clean” state if need be. Can you explain your question further?