This guide is based on Yaboot and powerpc machines. If you have no uses for a powerpc machine then this will be of little use to you.
Yaboot is the linux utility, and I believe BSD?, for powerpc machines. Before talking about this extensively, please do not try to use linux on a low end powerpc machine. On my iBook G4 (1.07 GHz, 1.25 GB ram, ati 9200) linux does not work properly. I blame the lower binned G4 chip. If you have a 1.33 GHZ ibook or higher or a 1.5 GHz powerbook or higher then linux should be fine for you. If you have ANY G5 machines then they are the perfect linux candidate.
Now then, yaboot is a simple utility. Often Yaboot comes with pre-determined boot categories but gives you room to write your own arguments. By typing HELP you can get a guide on how to boot by hand in yaboot. Often there is a pre-determined "Live" or other arguments for you to choose from. Many Args give you different situations as well, like if you don't have a mac GPU in your G5 but you want to make an NV 9800Gt work later on you can boot and install CLI only. Though often you can just hit enter and get into a live environment.
At this moment, similar to my post on installing windows, I am completely unsure of if you can install Grub on a PPC machine. I believe it just runs through the EFI and figures it out on the way but it is hard to tell with these old machines. I don't know much about Yaboot so if anyone on the forum has more information feel free to post!
Notes
This may be in my testing with a low end iBook and a screwed up powerbook, but every time I booted Ubuntu 14.04 and up the drivers would not work no matter what even though my powerbook has a standard 9700!! Be aware that if you have an odd chip like an ATi 9200 or 9400 then your system may not run very well at first. In my case the radeon drivers just crash out and I can't do anything. I don't personally know where to look but as far as I know unless you choose CLi in Yaboot you're screwed, at least with my model.