Tuaw has a great tutorial on how to install Ubuntu Linux on your Mac machine. This uses Sun’s VirtualBox, which essentially lets you install any kind of operating system. This is a great way to have another operating system under your main machine. Of course, this isn’t a real install since it’s virtual but it still is a great way to have another operating system on your Mac without re-installing your main OS.
You can also use this on PCs in a similar way and works just as well FYI.