The wind and the water, both rather limited to specific activities (milling, sailing). And the power of human and animal muscle. Where the animals are stronger, but also much dumber, so most of the actual hard work has to be done by human hands.
Basically all the settled civilizations used some sort of non-free or at best semi-free labour. Villeiny, serfdom, prisoners of war, slavery of all sorts, or having low castes do the worst work.
And given that humans are very good at rationalizing away their conditions, the cultures adapted to being comfortable with it, even considering the societal inequality as something ordained by the gods or karma.