There are a number of successful and popular Pinewood Derby designs, including the "Bond" style, an aerodynamic wedge with plenty of room in the back for weights, and the "Fonz" style, with a rear spoiler and unique weight distribution.
All designs are available for free download in PDF form from the site, although some of them are more detailed than others. The most basic blueprints show the exact size of the car and where the cuts are. The most detailed ones show step-by-step how to build the vehicle.
Boysdad.com claims that one of the most successful designs is the "Flying Wedge" style, which is a sleek, thin wedge designed for good rearward weight distribution.