Raspberry Pi Serial BoB

This board is designed specifically for the Raspberry Pi This board breaks out the I2C bus, the two SPI busses, and the UART. This enables you to connect our expansion boards to your Raspberry Pi, to easily interface with the outside world!
It is possible to connect 5V devices to this board. The I2C bus has a full level shifter installed on this board. For the TX and MOSI lines, the 5V device will recognize the 3.3V from the raspberry pi as "high" and function properly. For the RX and MISO lines this board has a protection circuit: a series resistor and a diode to the 3.3V powersupply line.
Datasheets:
Documentation for the complete PCB