A Seed Spreader.
The Seed Spreader can be purchased from Pitchfork Jeb's Farmer Supply. To use it, go to your farm, select it, and click place. Once on your farm, drag your desired seed bag to it in order to fill it up. Then use the action button to grab it and walk over your tilled ground, it will plant your seeds. To remove seeds, click the seed bag icon over the spreader after you grab the spreader. The seed bag will then fall on top of the spreader for you to grab once you let go of the spreader.
The Seed Spreader seeds three plots at a time.