How to set up Ubuntu Linux on a Mac

How to set up Ubuntu Linux on a Mac -- it's easy and free: "I received a private tweet from a reader the other day, who asked a question about a previous post in which I had installed Windows 7 RC1 on a Mac using Sun's VirtualBox. The tweet asked, 'You did a post about installing Win 7 on the Mac for free. How about Linux?,' which set up a challenge. Not only can you install Linux distributions on a Mac using VirtualBox, but it's extremely easy to do."