There's something about the Arduino that brings out the strangeness in people, and that's pretty much the only way we disguise this stacked monster. Dubbed the Core3duino by its creator, John David, it takes and existing, dual-processor mod for the Arduino and adds in a third CPU. Not only that, you also get a full 60 I/O connectors.
That sixty is made up of 24 digital I/O pins, 18 pwm pins and 18 analog inputs, along with six external interrupts. Even though you've already stacked two boards on top of the base Arduino, you can still add a further one (which will also plug into the base) such as a network shield or similar.
As for why you might do it, there are plenty of advantages to having extra CPUs during testing. John David himself describes the extra security in terms of WAF:
"While testing my larger bots, I always use 2 Arduinos to decode the drive and failsafe r/c channels separately. That way if something goes way wrong or the code doesn't work as it should, you have a way to shut the thing off before it busts a hole in the wall and your wife tells you no more inside testing"
[via Hack a Day]