Car sun shades are very useful. You can put them up when you leave your car to keep the sun out. This keeps your car cooler, reduces the amount of gas you use slightly (because you don't need to run your air conditioner so much to undo the heating), and helps protect your seats and dashboard.
The problem is that they are manually operated. The popular silver shades are unrolled and placed in the windshield. There are also black perforated shades that can be pulled out. They're manually operated because it's too expensive for third parties to automate this stuff into every type of car.
Carmakers should automate sun shades. When the car is off, you should be able push a button to have a sunshade come up. For the windshield it could be set up just like the windshield wiper—an arm comes up and spreads out a folding shade. For the side windows it could be like the power windows—a reflective cover comes up from the door. Someday we'll use windows that can become opaque. Right now it's too expensive, but maybe in five years it will be reasonable.
If we had automated sun shades, far more people would use them.
Update: [2006-07-30] These car shades are a step in the right direction.