When you get in a crash, something needs to decide whether or not to deploy airbags in your vehicle. Airbag deployment decisions are decided by a microprocessor that runs a program in ROM and receives sensory input from a MEMS accelerometer, which measures changes in velocity. In some vehicles, this system has become known as a "black box," even though its highest priority is to decide whether airbags (and seatbelt pretensioners) should be deployed.
Why the term "black box"? Because some manufacturers (currently General Motors and Ford) have included a few bytes of memory in the black boxes that record data on vehicle speed, brake usage, throttle position, and engine RPM in the seconds prior to a crash. Say a 1999 Chevrolet Tahoe gets in a crash severe enough to cause airbag deployment. For five discrete seconds before the crash, someone (e.g. an accident reconstructionist) can find out how the Tahoe was being driven before the crash.
This works two ways. If you get in a wreck and you were driving fine, you can use the black box data as evidence of that. If you were driving like crap, it could be used against you in court. People have been convincted across the country based on black box pre-crash data. The black box has withstood all sorts of legal challenges (based on technical and civil rights issues). Nevertheless, you'll occassionally see "shocking" news reports on black boxes spying on driving habits, which they've been doing for about a decade.