TechEBlog has a list of the Top Ten Strangest LEGO Creations. My favorite creation of the bunch is the LEGO difference engine. that thing is just crazy cool. (and TOTALLY geeky)

From Wikipedia:

A difference engine is a historical, mechanical special-purpose computer designed to tabulate polynomial functions. Since logarithmic and trigonometric functions can be approximated by polynomials, such a machine is more general than it appears at first.

-Matt