The cable USB to TTL Serial Cable is easiest way ever to connect to your microcontroller/Raspberry Pi/WiFi router serial console port. Inside the big USB plug is a USB <->Serial conversion chip and at the end of the 36" cable are four wire - red power, black ground, white RX into USB port, and green TX out of the USB port. The power pin provides the 500mA direct from the USB port and the RX/TX pins are 3.3V level for interfacing with the most common 3.3V logic level chipsets.
Because of the separated pin plugs, this cable is ideal for powering and connecting up to the debug/login console on the Raspberry Pi. Connect the pins as shown to power the Pi and establish the RX/TX link. Then install the Windows XP/Vista/7 or MacOS X PL2303HA/HXA drivers on your computer and connect with a terminal program at 115200 baud.
Also handy for hacking WiFi routers to install alternate OS's, or nearly any other TTL port. This is easier to use than an FTDI cable in many cases because the wires are separated.