I know that water expands in the freezer, but I'm curious about which materials contract in response to cold temperatures --- and most importantly, which ones undergo the most drastic changes?
Answer
Most materials contract on cooling. The notable exception to the rule are some phase transitions and water. But even ice contracts on cooling. Water expands on cooling only between $0^\circ\text{C}$ and $4^\circ\text{C}$ (including phase transition). This corresponds to the part of the graph below, in which density rises with temperature (note suppressed zero).
As regarding to what material contracts most, what are you looking for is the coefficient of linear thermal expansion $\alpha$
$$\frac{\text{d}L}{L} = \alpha \Delta T.$$
There is plentiful of various tables available on web, e.g.
http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html
It seems plastic materials contract most on cooling. Ethylene ethyl acrylate (EEA) for example has the largest one coefficient among the solids in this table.
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