FTDI Cable 3.3V
This a practical and easy to use FTDI cable. If you look carefully at your Ardupilot board you will see the labels "BLK" (black) and "GRN" (green), those colors indicate the polarity of this cable. All the FTDI stuff is inside the USB connector, making it look like a regular USB cable.
Note: The TX/RX signals are 3.3volts, making it even more flexible, the power supply is 5volts (red wire).