If you want to build a windmill, there's usually nothing in the game that lets you build an animated one. With this mod you can add rotating sails and blades.
Included: Classic windmill sails idle, classic windmill with sails set, modern three blade version, slightly less modern four blade version, classic american movie farm windmill. They can all rotate clockwise or counter-clockwise.
If you want to use larger windmill sails/blades, take a look at windmill_large.