Redstone.
And yeah, I do now remember someone was asking a while ago if a
turing machine was possible, which I think it is - so with enough time, space, and resources anything can be emulated.
Of course, input and output methods are limited, and running anything complicated inside a game inside a Java VM inside an OS is not going to give the greatest performance.
But it's still cool how this "simple" stuff can allow such complexity.
EDITED: 13 Jan 2011 16:41 by BOUGHTONP