You can read everywhere about water's extraordinary property of expanding when frozen, thus the reason ice floats on liquid water.
What other substances do this? There are claims of mercury, silica, germanium, bismuth, and antimony, but I've had trouble tracking down the data to back these up.
Wikipedia quotes
Other substances that expand on freezing are silicon, gallium, germanium, antimony, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination.
EDIT:The same paragraph says silicon dioxide also exhibits this property.
