The average human’s resting heat dissipation is something like 2000 kilocalories per day. Making a rough approximation, assume the brain dissipates 1/10 of this; 200 kilocals per day. That works out (you do the math) to 9.5 joules per second. This puts an upper limit on how much the brain can calculate, assuming it is an irreversible computer: 5 *10^19 64 bit ops per second.
Considering all the noise various people make over theories of consciousness and artificial intelligence, this seems to me a pretty important number to keep track of. Assuming a 3Ghz pentium is actually doing 3 billion calculations per second (it can’t), it is about 17 billion times less powerful than the upper bound on a human brain. Even if brains are computationally only 1/1000 of their theoretical efficiency, computers need to get 17 million times quicker to beat a brain.
The model of the Antikythera mechanism shown here is one of the clearer I've seen. I'm fascinated by analog computers, but figure we need an entirely new category for "organic" models.