This is a shield for the Arduino that has a seperate Atmega168 microcontroller built onto it with it's own crystal oscillator and power regulators. You can power it with the arduino that it is plugged into using onboard jumpers.
Using I2C, you can connect 1 Arduino (master) to a host of slave Arduino's through Analog ports 4 and 5.
Also, you can add the security of having a completely separate CPU to your project that is unaffected by any code running on the base Arduino. I use this shield as a failsafe on my R/C lawnmower. The base processor uses the 2 external interrupts to sample and decode 2 servo signals from an R/C transmitter/receiver, while a 3rd servo signal is sent to the Core2duino that controls a relay for the motor-controller power supply. This way, even if there is a problem with the main code and it stops responding, the Core2duino will still be able to carry out it's main function unaffected (which is to kill the power to the bot if anything unusual happens).
I have included the Eagle files needed to build the board. It is basically just a breakout board for the Atmega168, that has it's own power regulation and crystal oscillator. It provides access to the base Arduino's pins by using stackable headers that plug directly into the base Arduino.
It is a fun board to play with and I thought I would share, just in case anyone else has a use for it.
Here are some pics: