Water wheel

One of the best labor saving, and labor magnifying devices ever conceived, the water wheel benefited humankind in many ways.
It allowed for a greatly expanded range of places for agriculture. No longer were farmers tied to the reach of the local river or stream, or entirely dependent upon the sometimes capricious whims of nature and the rainfall. Now they could move water where they needed it, when they needed it. Of course if the water became so scare that the river or stream also dried up, there still wasn't much they could do.
It also found purpose in the results of that increase agriculture, the grinding of grains. The power of the water drove one great stone wheel against another stationary lower stone wheel, and flour of a uniform consistency was had. The mind-numbing and endlessly repetitive process of hand grinding was no longer needed.
With increased agriculture, and increased access to both water and food, the population was able to grow. All these new people needed employment and places to live, and the water wheel assisted there as well, becoming the power that drove the sawmills, turning logs into planks. It became unnecessary to do this dangerous work by hand. While many could be employed at a sawmill, it was also much more productive and produced a more uniform plank that those cut by hand.
Moving water, grinding grain, cutting logs - these are only a few of the tasks a water wheel could perform. Certainly even more uses will be found in the future.

Utility

The wheel can be for providing grinding power, turning grain into flour, or lifting power,  or transformed via gears to power a sawmill.
It is also used to lift water from the river or stream into a canal for irrigation purposes.

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Access & Availability

Currently it is a generally well known device, used wherever there is flowing water, or a means of making flowing water.

Complexity

They can be simple, just a wheel with paddles and a means of braking or removing it from the flow, to complex geared devices.

Discovery

This device likely began as a child's toy, something that would spin when held in a stream or river. Likely the creative mother or father soon recognized that if it were enlarged, it could do many useful things.