You can put linux on it, either via a Live disk, or a hard drive install. When you are looking for a distro, just make sure that it is for PowerPC(32-Bit), and not i386/PowerPC64. There are PowerPC versions of almost all major distros (Ubuntu, Debian, Fedora, Ect.) You cannot run Windows on this computer, however.

*Also* You may need a DVD drive for some distros, since they may exceed the 700MB CD-ROM limit. You will need another computer (Doesn't matter what type Mac, PC, Ect.) just to burn the disk. I would recommend PowerPC Ubuntu 10, I run this on my PowerMac G4 MDD(Alongside OS X Leopard, on a second hard drive), and it runs pretty well.

Once you burn the disk, put it on the computer, reboot holding the "Option" key (Left-Alt on non-mac keyboards) and select the disk from the menu that comes up. You can also hold the "C" key on the keyboard instead of this to force-boot CD. Then, just follow the instructions.