Aiming to help users drive more efficiently, meanwhile, is a fuel-efficiency app that alerts you when you're driving too fast, accelerating too quickly or breaking too suddenly. The iPhone app is free. Users, meanwhile, report reductions in fuel usage of up to 16 percent, its makers say, translating into hundreds of dollars per year.

5. Avego

For further savings while on the road there's also , a free ride-sharing app for iPhone and iPad. Basically, you can use the app to offer vacant seats in your car to others for the daily commute, say, and then receive money from them to help offset the cost of your trip. The goal: Fewer cars on the road, fewer emissions, and savings for users.

6 and 7. Green Charging and Eco Charger

We're all familiar with the necessity of charging our mobile phones regularly; what many don't know, however, is that leaving your phone plugged in once it's charged wastes energy--and money.