The Toyota Prius (yes, the hybrid car, shown) tracks your gas mileage over time. As you drive, your driving habits (lead-footing it, slow acceleration, etc.) affect your gas mileage. Wired recently had an article about how different drivers' driving styles can affect their resulting MPG significantly.
So, I was thinking...what we need is a high-score uploader! Imagine that your Prius, using its Bluetooth technology, can connect to the Internet through your mobile phone. When it does, it uploads to a secure server your fuel efficiency for the last week, month, year, and the lifetime of the car.
Then, just like console and PC games, you can go online and check out how you fare compared to all the other Prius drivers out there. Or, you could compare yourself with all the other drivers in your state or your city (I'm guessing San Francisco drivers might be at either an advantage or a disadvantage due to the hills). Perhaps even better would be the ability to pull up the "high scores" right on the Prius' in-car display. Perhaps this would be the first actual reality-based driving game.
Suddenly, it becomes a competitive activity to try and get the most fuel economy out of your car. Every time you drive, you're trying to maximize your distance while minimizing your gas usage. Prius drivers who just want to "win" at the game end up saving gas and being environmentally friendly, regardless of their eco-political perspective.
No, this doesn't exist yet, but I think it would be a pretty cool idea. I know that it might just sell some more Priuses as well and would be pretty inexpensive to do.