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Core3duino is, believe it or not, three-core "Arduino cluster" in the form "an addon shield to a shield".
Core3duino has three Arduinos stacked one on each other. The core extension boards fits on popular Duemilanove boards and Mega boards as well.
If you decide to built it, you end up with:
3 separate processors
60 input/output of which:
24 digital I/O pins
18 pwm pins
18 analog inputs
including 6 external interrupts
...all for about $25 more than your Arduino (regarding author information).
Read more at Instructables: Core3duino