A breakout board that grants 8 additional digitial IO pins using the MCP23008 (I2C) or MCP23S08 (SPI) chip.

Both the SPI and I2C version contain 2 connectors, so daisy chaining multiple boards is an option.

Since it uses SPI or I2C, this board can be used by almost all microcontrollers, such as Atmel AVRs, Arduinos or PICs. A Raspberry Pi should also be able to control this module.

Datasheets:
Documentation for the complete board
MCP23008/MCP23S08 Datasheet