A small board equipped with an FT232RL USB to UART bridge chip. Both 3V3 and 5V IO operation is possible, and is selectable with a jumper.
The PCB is also equipped with three LEDs: A power-LED, and two LEDs to indicate Rx and Tx activity.The 4-pin header breaks out GND, Tx, Rx, and V+ (selected by jumper: 3V3 or 5V).

The 3V3 regulator can supply about 50mA for your project. (i.e. good for small projects, arrange your own 3.3V supply for larger projects)

Datasheets:
FT232RL
Documentation for the complete PCB