"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)."
I'll probably start with these simple instructions: Using VirtualBox to run Linux from within Windows.
I also plan to check out the VMKnoppix live CD.
(As if I need more stuff to do;)
I've been using Microsoft Virtual PC 2007 and notice a lot of similarities between it and VirtualBox. Maybe that's because they share a common ancestry. Virtual PC was originally developed by Connectix and was later sold to Microsoft (see Microsoft Virtual PC: Version History). VirtualBox came from Innotek which, before being acquired by Sun Microsystems, worked with Connectix on Virtual PC technology.
Rich Green: On Snowglobes, xVM and Freedom
Virtual Steve: Weblog: This Isn't Your Dad's Desktop Virtualization