You'll need miniature pumpkins (I used Baby Boo), unflavored gelatin, salt, vinegar and oil from the grocery store. You'll also need 18 gauge copper and aluminum wire, an LED (preferably one that runs on 1.5 volts- I used one that I had candied in an earlier project), wire cutters, pliers, a ruler and multimeter. In the kitchen you'll need a medium sized bowl, small bowl, small pot, cookie sheet, pairing knife and mellon baller (or spoon). If you already have the tools and kitchen stuff the project should cost about $20.
Update: One week later and the pumpkin battery is still generating 1.5 volts! My follow up post on it is here.