My iBook G4 is dual boot Lubuntu(not sure what version) and OS X.
Lubuntu was installed on the computer when I bought it-the seller asked if I wanted it left or wiped clean, and I said to leave it since it was a ready-made install for me to play around with it.
I've actually only booted to it once or twice, and didn't do a whole lot since the wireless drivers weren't configured and didn't feel like putting the effort into getting them working.
That particular computer came to me with Tiger installed, and I upgraded to Leopard after maxing the RAM.
The PPC computers that I actually use regularly primarily are running Leopard with a few running OS 9. I have several setup to dual boot Tiger and Leopard-mostly for classic mode support on computers that can't natively run OS 9-but rarely boot to Tiger.
Most of my G3 based computers have 10.4 installed, although I tend to keep G3s as dedicated OS 9 machines. The only G3 I have that both gets(semi) regular use and runs OS X is my Pismo Powerbook, which actually does relatively well with Tiger on the max of 1gb of RAM.
I have a closet full of G4 towers that rarely get used but are mostly running Tiger...primarily because they don't "officially" support Leopard, I haven't take then time to work around the installer, and most don't have enough RAM to run it well anyway(and I don't feel like dumping more money into them to put 1gb+ in each). There's also the graphics card issue, since I've pulled the "good" graphics cards out of most of the ones in the closet, and left most of them with Rage 128s-Leopard is not a pleasant experience on that card.
I should also mention that my pair of G4 towers that get used a lot for diagnostic and testing purposes-a Digital Audio and a Sawtooth. Both have Tiger and OS 9 installed, although the DA normally gets booted to Tiger and the Sawtooth to OS 9.
Then there's the MDD G4 that I loaded up with hard drives and have 10.2, 10.3, 10.4, and 10.5 installed on(every version of OS X that it supports) plus OS 9. I only ever boot it to 10.5, but wanted to get the full OS X experience. If I get around to it, I'm probably going to take a B&W G3 or one of my other early G4s and load 10.0 and 10.1 on it(plus 10.4) so that I have working installs of the full range of PPC versions of OS X.
While I'm at it, I'll also mention my 7100/66 running OS 8.5, my 8500 running OS 8(I probably should downgrade it back to 7.5.5), and my Quadra running 7.5.5. Once I finally get a working hard drive in my SE, I'm probably going to install System 6.