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#11 It's a frickin' tricorder
You can download a free app called WeatherSignal that provides measurements including temperature, light, humidity, pressure, acceleration, and even magnetic flux (measured in microteslas).
While there's some useful information to be gained by all these sensor readings, one thing I learned is that there's one heck of a lot of magnetic flux within a few feet of my big server. It's a good thing I don't sit in front of it all day. Fortunately, the Inverse Square Law applies, so my desk (which is about six feet away) has almost no measurable flux (unless, of course, I exceed 88 miles per hour).
Once again, the iPhone can't do this.
Image courtesy my phone and the WeatherSignal app.