I love car apps.
I regularly use the Waze app to find out what's going on with traffic (INRIX looks good too), Gas Buddy to find cheap gas, and RoadAhead to find out what food options are at the next rest area. I used to obsessively track my fuel economy with GasCubby until I got a car with the MPG on the instrument panel. I also love the iPhone-connected Cobra Atom radar detector, and commuting would be virtually impossible for me without the Downcast and Audible apps.
I just received my private beta Link hardware from Automatic (pre-order for $69.95) and am enjoying it immensely. Automatic Link (pictured) is a small hardware device that plugs into your vehicle's diagnostic/data port (usually located just below the dash) that tracks your driving habits and gives you tips on how to save money by driving more economically.
Link connects to the free iOS app over Bluetooth Low Energy and displays driving data in an elegant and easy-to-understand UI that tracks how many miles you've driven, driving hours, fuel usage (in dollars), and actual MPG, even for older cars that don't display fuel efficiency on the dashboard.
An exceedingly cool features is that Automatic detects fill-ups and tracks local gas prices (on newer vehicles) to show you how much you're spending on each trip in real dollars. Automatic combines all the data it collects into a score on a 100 point scale and I was able to score a 96 in my first test of the hardware.
In addition to the data provided by the iOS app, the Link hardware contains a small speaker that emits different sounds to alert you when you're accelerating or braking too fast, or speeding — which can have a negative impact on fuel economy. It's like having your wife driving in the passenger seat, 100 percent of the time! Of course, I jest. She actually reads these posts (and I like to eat dinner.)
In addition to its fuel-saving features, Automatic has several OnStar-like features, minus the recurring fees. The Automatic Link includes an accelerometer that can detect many types of crashes and can use your phone's data connection to report a crash to 911* with your name, location, and vehicle description. After help is dispatched, Automatic can send a text message to a loved one to let them know what happened, where you are, and that help is on the way.
Automatic can send you a push notification when the dreaded "Check Engine" light comes on and can decode the cryptic Engine Trouble Codes generated by your vehicle and offer possible solutions. For simple problems, you can even clear the light yourself saving a trip to the shop. It can even help remember where you parked, which I could have used when searching for my car in the long-term parking lot at midnight last night after a week's vacation.
Check to see if your vehicle is compatible and find more answers on the Automatic QA page.