Ubuntu Linux, I use to be a slackware user, and then got married. I have to spend time with my wife, instead of fiddling with stuff like adding middle mouse roller to the X.org install.
Leopard Runs on my PC, with stability issues with my setup. Mileage will vare from setup to setup. Someone in a earlier post said you can't use updates, but that isn't true anymore. Applying an EFI patch(which needs a system somewhat close to an actual mac hardware, ex: intel motherboard p33-p35 type, core duo processor), and if Im correct a you can use vanilla(non-changed) kernel. Otherwise, your going to be installing each update manually a bit different.
Its just of matter if you know your hardware and be ready for some major headaches. Plus, it use to be a lot worse, with technical knowhow, but oddly Leopard was probably the most easiest Mac OSX to install to date.
For headaches for me For example, I can't run Open Office. That really makes work harder for me.