PyPI is described as 'The Python Package Index (PyPI) is a repository of software for the Python programming language. PyPI helps you find and install software developed and shared by the Python community. Package authors use PyPI to distribute their software' and is a Package Manager in the development category. There are more than 50 alternatives to PyPI, not only websites but also apps for a variety of platforms, including Linux, Windows, Mac, Python and BSD apps. The best PyPI alternative is Synaptic, which is both free and Open Source. Other great sites and apps similar to PyPI are Spyder, SciPy, npm and Anaconda.
PyPi offers Python packages while Synaptic deals with linux packages. PyPi is a repository and not an application, while Synaptic is a GUI application and not a repository itself.