MOPAC (Molecular Orbital PACkage) is a semiempirical quantum chemistry program based on Dewar and Thiel's NDDO approximation. Available at this site are the open source version (OpenMOPAC) and information on the commerical version.
cclib is an open source library, written in Python, for parsing and interpreting the results of computational chemistry packages. The current version parses output files from ADF, GAMESS (US), GAMESS-UK, Gaussian, Jaguar, Molpro, ORCA and Firefly.
Molden is a package for displaying Molecular Density from the Ab Initio packages GAMESS-UK , GAMESS-US and GAUSSIAN and the Semi-Empirical packages Mopac/Ampac, it also supports a number of other programs via the Molden Format. Molden reads all the required information from the GAMESS / GAUSSIAN outputfile. Molden is capable of displaying Molecular Orbitals, the electron density and the Molecular minus Atomic density.
Gabedit is a graphical user interface to computational chemistry packages like Gamess-US, Gaussian, Molcas, Molpro, MPQC, OpenMopac, Orca, PCGamess, Q-Chem and DALTON.