It allows you to turn an AVR micro into an I2C slave chip.
$29
You could start your own factory for I2C chips: Create your own PCF8574 , I2C LCD display etc. Works with 2313,2323, 2333,2343,4433, Tiny22, Tiny12, Tiny15 and M8. Other AVR chips have built-in hardware for I2C. Chips with built-in TWI are supported with a special I2C-TWI library that is shipped with the I2C slave library.
