The free GoPoint app works with the GoPoint cable (which costs $100, but can be purchased online for less) to plug into your car's On-Board Diagnostics (OBD) port -- don't worry: Every car manufactured since 1995 has one -- to give you real-time information from the vehicle's built-in computer diagnostic system. This is the sort of thing that your local mechanic will do using a piece of equipment that cost thousands of dollars, but your iPhone can do it for the cost of a cable and the time you spend downloading an app. Some people would consider that a bargain.
What sort of information can the GoPoint app extract from the OBD port? Well, if you already know that your car is experiencing some kind of problem -- for instance, if the Check Engine light is flashing -- the GoPoint app can give you the Diagnostic Trouble Code (DTC) for it. These are standardized codes and the GoPoint app will define them for you in plain English. Unless you're a mechanic, you might not be able to deal with the problem itself, but you'll know what to expect when you take your car into the shop and can verify that your mechanic isn't just inventing expensive problems that will cost you a fortune to fix.
Oh, yes -- once you've purchased the GoPoint cable, there are additional apps that work with it, not just from GoPoint but from third-party developers, so that you can do things like monitor your fuel economy in real time.
The final iPhone app on our list has a funny name, but after you read about what it can do, chances are you won't be laughing.