Solid State: Playground and Miller indices

h:  k:  (-(h+k): ) ℓ:
You can also load your own coordinate file. Jmol accepts several formats (xyz, pdb, mol,...), but cif is more appropriate for solids.
Several crystal structures of organic, inorganic, metal-organic compounds and minerals, excluding biopolymers can be downloaded on COD, an open-access site

More information about Miller indices on Wikipedia