There is a LEGO Turing machine, constructed at Aarhus University (see here for more information):

There is also a LEGO Babbage difference engine built by Andrew Carol:

Now we also have an Antikythera mechanism (an ancient Greek computer for predicting eclipses) made of LEGO, again by Andrew Carol:

(Thanks to CompSciFact on Twitter for the last one.)

I hereby propose that LEGO brick constructions are to be considered serious unconventional computing devices, and should be investigated by theoretical computer scientists in order to establish their computational and complexity theoretic properties. (What about LEGO bricks as a non-uniform computing model?)

#### Update (22 June 2012)

Here is another Turing machine built at Centrum Wiskunde & Informatica (Amsterdam) “in honor of the Alan Turing year 2012”:

And what about circuits? Here is how to construct mechanical (“push-pull”) logic gates. For instance, this is an AND gate: