Eclipse is an open-source community project that is focused on building an extensible development platform: a suite of software tools that assist in the writing of software. Primarily it is used as a Java integrated development environment (IDE) but has been adapted for many other languages including python, C, C++, and FORTRAN.
SciPy is open-source software for mathematics, science, and engineering. The SciPy library depends on NumPy, which provides convenient and fast N-dimensional array manipulation. The SciPy library is built to work with NumPy arrays, and provides many user-friendly and efficient numerical routines such as routines for numerical integration and optimization. Together, they run on all popular operating systems, are quick to install, and are free of charge. If you need to manipulate numbers on a computer and display or publish the results, give SciPy a try!
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.