This site lists 570 places across the U.S. that offer free tours. http://www.factorytoursusa.com/
It's supported by travelers who find and submit the information to this site.
Click on your state to see what's near you, or look for tours in other states where you plan to travel.
It's not a fancy site, but it's loaded with lots of interesting places to visit. All the entries have a map to get there, a detailed description of the tour, and a link to the company website.
For example, you can find interesting places to visit like these:
• Al's Family Farm in Fort Pierce FL, where you can see how they process Indian River Citrus, http://www.alsfamilyfarms.com/
• Check out the Cape Cod Potato Chip Factory in Hyannis MA, http://www.capecodchips.com/
• Get a first hand learning experience at Herr's Snack Factory in Nottingham PA. You can schedule your tour online at http://www.herrs.com/SnackFactoryTours.html
• You can watch chips fry and sample them at the Route 11 Potato Chip factory in Mount Jackson VA, http://www.rt11.com/viewing