PowerPC you have Leopard or Tiger, the latter being "faster" but more limited. You can also install the version of Leopard someone added Snow Leopard beta parts to, which I do have on my PPC MacMini, but it still gives you limited returns.
With PowerPC you might be better looking at a third party. I hear good things about MorphOS, though never tried it. You could do Linux too.
With PowerPC you might be better looking at a third party. I hear good things about MorphOS, though never tried it. You could do Linux too.