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.
PyQuante is an open-source suite of programs for developing quantum chemistry methods. The program is written in the Python programming language, but has many "rate-determining" modules also written in C for speed.
NWChem is an electronic structure package that features MC-SCF, MPn, CC, CI, and DFT methods. Properties, solvation models, QM/MM, and MD simulations are also possible.