Free open-source portable distribution of Python for Windows targeted at educational and scientific usage.
Ranked in these QuestionsQuestion Ranking
Pro Many integrated packages
Just to name a few: NumPy, SciPy, Sympy, Matplotlib, Pandas, pyqtgraph, cython, Jupyter
Pro Free and open-source
It's released with the MIT license. Sources are available on GitHub, distributions on SourceForge.
Pro Portable (but can be registered)
Can be installed without any integration into Windows. This makes it possible to have many different installations in parallel (Python 2 and 3, 32-bit and 64-bit). If needed, there is a Control Panel that let's you register a specific installation to Windows.
Pro Package Manager
The WinPython Package Manager provides a useful GUI to install or update packages. Naturally you can also simply use pip.