Most materials become more dense as they cool, but ice floats in water, meaning that it's less dense. Why is this?
Under normal conditions (meaning your regular everyday freezer, or a cold day) water forms a crystal lattice below its melting point (as shown below). This structure is held together by hydrogen bonds, which are no longer overcome by the internal energy of the molecules. The hexagonal arrangement of the water molecules in this structure leads to a lower density due to the spaces left in the center of each hexagon (you can now fit fewer molecules in the same volume).
There are many other solid phases of water (at least 16 on wikipedia), some which have densities higher than normal liquid water, though most are not naturally formed (at least on Earth).