Jmol

Jmol is an open-source Java viewer for chemical structures in 3D with features for chemicals, crystals, materials and biomolecules.

This app can read a multitude of file types, including PDB, CIF, SDF, MOL, PyMOL PSE files, and Spartan files, as well as output from Gaussian, GAMESS, MOPAC, VASP, CRYSTAL, CASTEP, QuantumEspresso, VMD, and many other quantum chemistry programs.