This is a “skeleton” of a page that I want to write about various versions of Python that are optimized for various applications. However, I now recommend that you just check out this article about all the different implementations of Python (CPython, Jython, IronPython, PyPy, etc.) by Charles Marsh.
Stackless Python is an implementation of Python that adds lightweight microthreads with scheduling and interthread communication. It’s aimed at concurrent programming. Note that Stackless microthreads do not take advantage of multicore or multiprocessor architectures. The threads all run within one Python process.
Jython is an implementation of the Python programming language that runs on the Java Virtual Machine (JVM).
PyPy is an alternative Python interpreter than incorporates a just-in-time (JIT) compiler that optimized bytecode to optimize performance.
Cython is a language that allows you to write C extensions for Python in a language that is very much like Python.