Simple wool farm

A one sheep wool farm by Triloms, and the smallest thing in the library that still counts as a farm. A single sheep stands on a grass block in a glass box, an observer watches for it eating the grass, and that fires a dispenser holding shears. The wool drops onto a hopper minecart parked on a rail underneath and goes into a double chest. Ten items, no redstone beyond one dust, and it tiles sideways, so most people build a row of them and put a differently dyed sheep in each one.

The gamerule mobGriefing has to be on. Sheep cannot eat grass without it, so nothing ever triggers. ryanfisk9989 works around it by powering the dispenser from an observer clock instead.
Do not break the rail. 594Icestorm says the hopper minecart does not sit high enough to reach the wool without it, which is behind most of the reports of wool lying on the ground.
Shear the sheep once by hand to start it off, and if nothing happens turn the observer the other way round. blackwood6992 also catches people using a dropper instead of a dispenser.
It needs light. Grass will not spread in the dark, so an underground build wants a light source over the dirt and Connor_Tyler adds torches outdoors so it does not stall every night.
Nine shears fills a double chest before it runs dry. Whoseknownasthis makes it truly endless by feeding an iron farm through autocrafters into the dispenser so the shears replace themselves.
HeadHoncho727 swaps the hopper minecart for an ordinary hopper and chest to cut lag, because the game keeps ticking the cart even while it sits still.