Place peanuts in a food processor. Turn the food processor on and let it run for 4-5 minutes. During this time, you'll see the peanuts go in stages from crumbs to a dry ball to a smooth and creamy "liquid" peanut butter. Stir in the sugar, honey, or salt, if you want.
Store in the fridge or at room temperature if you think you'll go through it fast enough (I know people who store it both ways - I would just advise the fridge if it's going to be several weeks before you get through it).
Recipe by Pinch of Yum at http://pinchofyum.com/5-minute-homemade-peanut-butter