The Apple iPhone has a built-in accelerometer that rather accurately detects even the slightest motions. Developers have been quick to create applications that capitalize on the iPhone's hardware, including software that helps enthusiasts obtain g-force and 0-60 times from their phone. We've tested a few ourselves and they work pretty well, although most of these apps come at a cost, ranging from $4.99 to $10 or more. If you like the idea of timing your vehicle with the aid of an iPhone but don't want to pay for an app, BMW has a free solution for you.
It's called the M Power Meter and does most things those paid apps can do, but it's free and also very easy to use. Simply download the software from the app store [iTunes link], place your phone upright in a cup holder or flat in the direction you intend to drive, hit start, accept the terms and go. The M Power Meter defaults to timing a jaunt from 0-50 km/h, but you can change that by clicking on settings and choosing from several acceleration options up to 200 km/h or distances ranging from 100 meters to 2,000 meters. We downloaded the M Power Meter app and found it worked fairly well for being free and nearly as good as those apps we spent a few bucks on in the past.
If you have an iPhone and a car, test it out for yourself (without breaking the law or doing anything that could endanger yourself or others, natch) and tell us how well it worked by heading over to the Comments section. It's free, so it couldn't hurt to give it a whirl.