VMWare (http://www.vmware.com/products/fusion/) virtualizes Windows or Linux on intel-based Macs:
Sponsored by VMware, the normally $10 ebook by Joe Kissell and the 'Take Control' book publishers is now a free download:
Take Control of VMware Fusion 3 (http://www.takecontrolbooks.com/vmware-fusion-3)
Plus
this ebook comes with a coupon worth 50% off your next Take Control (http://www.takecontrolbooks.com/catalog) order.
You can listen to Joe Kissell discuss this book as episode 9115 at macvoices.com (http://www.macvoices.com/wordpress/macvoices-9115-joe-kissell-takes-control-of-vmware-fusion-3-for-free/)