Molden
An open-source application for pre- and post-processing for quantum chemistry calculation. This application can handle outputs from Gaussian, GAMESS, and MOPAC as well as the result of other applications via the Molden format. It supports many graphical interfaces such as Postscript, XWindows, VRML, and OpenGL, and performs visualization of molecular orbitals and electron density. It also produces animation videos of molecular vibration.