Ubuntu: I'm sure you can, but this is not the place to ask about it. I suggest going straight to the
source for info on how to do it.
Mac OS X Tiger: It has been done, with hacked versions of the OS. It usually doesn't work all that well, with things like sleep not working like it should, but it's been done. You need a copy of x86 10.4, however, and those are not possible to buy (without buying an Intel Mac of a certain generation). There are probably places to download them, but I don't know where and I wouldn't be able to tell you if I did - that's copyright infringement, and illegal. Newer versions of the OS can at least be obtained legally, and have also been made to work.