The Nap Cap ($99.95) is designed to stabilize your head while traveling in cars, buses, and planes, so that it is easier to sleep normally, and avoid a stiff neck upon waking up. 

The nap cap is actually only part of a product line that also includes the Nap Strap.  These comfortable, soft, washable nylon products come with a whole slew of straps, adjustable and expandable, to fit the cap to your head as snugly as possible. The package also includes easy to follow instructions, soft ear plugs, and removable fleece eyeshade complete in a velvet carry pouch to make your trip convenient. Although they are a wonderful idea, the products do make you look a bit ridiculous, and have had mixed results among the FTF tester families.

This reviewer found that they all do work.  Although my head wasn't flopping around, the feeling of having it immobilized by Velcro made the concept of sleep hard to visualize.  I suppose if used regularly, this problem could be avoided.  Our other reviewer, however, found that the child's cap was too small, but the adult one was too large.  Also, they noted that unless the straps are anchored correctly, the sound of Velcro slowing becoming undone could get annoying.  Clearly, these products have met with some disagreement from testers, but if you wish to have better sleep on a plane or car, this product is for you. Product comes in three different colors and are available in large and small sizes.