This is a links I should review as soon as I have time: PowerBook with Several OSs. Because one of the things I should do soon is to install other operative systems in my iBook. (PowerBook and iBook should be similar).
Finally the link was not what I have expected: instead of explaining how to install alternative OS in a PowerBook, it only describes how to run different OS in an emulator (Virtual PC). I was not interested in that, I wanted to run native PowerPC versions of the systems. (Although I enjoyed the short description and screenshots of different OSs)
But on the same site, I discovered a great article on What is Mac OS X?. I know it may sound a little bit shameful, but I knew very little on the MacOS X although I use everyday a copy of it. When I start the Terminal.app it seems to me I am in some kind of *nix environment, but with some strange discrepancies: for example, no /proc filesystem, filenames where not case sensitive, links continued to work even after moving the files,... This short article helped me hugely to understand better what really is MacOS X.