An example of this is a crystal like diamond or zinc sulfide. Covalent crystals can have extremely high melting points. Metallic crystals normally have high melting points and densities. Ionic crystals are hard and have relatively high melting points. An example of this type of crystal would be sugar.
Molecular crystals tend to be soft and have lower melting points.
Of course, this classification scheme has some ambiguous areas. I'll write about both. If the molecules come together in a random arrangement, they do not occupy the closest packed space. Thus, the proper packing uses less space and is also of lowest energy, which is always the most stable condition. As it happens, this ordered array pattern repeats itself regularly in 3 dimensions, and the crystal is the macroscopic object we see as a result.