Protein crystallography is a very hot field in the biosciences these days. There are a couple of ways to answer this question. I generally think in terms of crystal systems and lattice types. So really a single crystal of a covalent crystals is really just one big molecule. 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.