Toyota Prius

The engine of the Toyota Prius turns off when you stop, so it's easy to forget that it will move when you press the accelerator. As a safety measure, Toyota installed a loud beep in the interior when the car is in reverse. Some people find this annoying. Luckily, it is easily disabled. This works best with earlier models.

[edit] Steps

Press the power button with your foot on the brake to turn the car on.
Press the trip/odometer button until "ODO" appears on the dashboard (if it's already in that mode when you start the car, press the button three times to change to the trip mode and back to the "ODO" mode).
Press the power button to turn the car off.
Start the car again by pressing the power button with your foot on the brake.
Press and hold the trip/odometer button within six seconds of starting the car. Continue holding the ODO button for at least 10 seconds.
While still holding the ODO button, shift the car into reverse and quickly press the park button. Now release the trip/odometer button.
Note that the trip/odometer display will show "b on" instead of miles.
Press the trip/odometer button until the display shows "b off."
Press the Power button to turn the car off.
When you start it back up, the reverse beep will be gone.

[edit] Tips

To reenable reverse beep repeat the procedure. "b off" will be displayed in step 7. Toggle it to "b on" in step 8.
Please note that this procedure will work ONLY on a North American model Prius, not on any other model.
This procedure will not work if a passenger (or any other object) is sitting in the passenger seat.
To disable seatbelt beep, use same steps as above, but instead of shifting, fasten seatbelt. A passenger (or any other object) sitting in the passenger seat is required for disabling passenger seatbelt beep.

[edit] Warnings

The beep is there for your safety. Be aware that if you turn it off, you will not be warned of backing up.
Whether you disable the beep or not, you should be aware that it is only audible inside the car. It will not warn pedestrians that you are reversing your silent car.