Install Debian Etch on an older G3 iMac
One thing that stops many people from trying Linux is that either they don’t want to go through the hassle of dual-booting their machine and possibly loosing everything in the process, or that the ease of use with virtualization technology such as VMware and VirtualBox make non-commitment to an OS trivial. If one is serious about using an alternative OS, I think it’s better to use it on a machine all by itself.
What better way to do that than to get a cheap computer, in this case an old iMac, and load Linux on it. While the process isn’t exactly the same as doing this on an x86 PC, once the OS is loaded one gets a feel for how everything works. You’ll also get the advantage of a real, multitasking OS instead of MacOS 9 or earlier.