Wood frogs (Rana sylvatica) can freeze solid in winter, such that their heart– and everything else– stops cold… literally. During hibernation, the glucose in a wood frog’s blood is concentrated in the water surrounding its organs. The water doesn’t completely freeze (which would cause major cellular damage) because the high solute concentration lowers its freezing point. The glucose acts as an antifreeze or cryoprotectant. This trick is used by other animals that must regularly cope with subfreezing temperatures, like the European common lizard (Lacerta vivipara). Other organisms use glycerol or antifreeze proteins.
This is just amazing, in my opinion. Check out this NOVA segment on the freezability of wood frogs!