The official Pinewood Derby website is a good starting place, as it has both rules and kits for Pinewood Derby cars. Pinewood Pro also has advice about rules, in addition to providing parts, kits and designs. Another official site is Derby Monkey, which offers kits, designs and helpful articles.
Boys' Life has a hobby section dedicated to the Pinewood Derby, including tips on painting and optimizing speed. Boys' Life also features interesting-and-unusual cars from previous years to help readers with design ideas.
As various sites mention, local Pinewood Derby organizations set specific rules for a race. The Boy Scouts of America rules form general guidelines, but Cub Scout packs do not have to follow those rules. Other organizations, such as the Girl Scouts or YMCA, also sponsor derbies with different names, as Pinewood Derby is a Boy Scouts' trademark. The different organizations, though, follow the same overall structure of weight and wheelbase requirements enforced by inspections. Official products are stamped with the organization's initials or name.
The basic Pinewood Derby car starts with a precut pine block, four wheels and nails for the wheels. Cars must include those parts, although builders may shape or carve the pine block. Other features are up to the builders' imaginations, within weight-and-size limits.