If you want a Mac OS:
604/120: 8.1 or 8.6 if you can find it cheap
'040/33: Mac OS 7 is availible for download from Apple's website. You have to look though, it's buried
If you want a *Nix:
604/120: Gentoo, Darwin, Debian, LinuxPPC (sorta dead), Mandrake, NetBSD, and Yellow Dog Linux all have PPC versions. Check out the system requirements and make sure you download the PPC version.
'040/33: No version of Linux supports non-PPC processors
486: Any version of Linux that says i386. IT HAS TO SAY i386+, not i586 etc...those versions are too high for a 486. Go to LinuxISO.org, and in the lower right hand corner there is a box, select i386 only.
Have phun with Linux!
Go here to download Linux:
www.linuxiso.org[ October 07, 2002: Message edited by: The_Muffin_Man/B0b ]