I have an X11 programme that I'm mostly using under Mac OS X (10.6.8) and for which I wrote a plugin to extend it with a Python interpreter, of course extending Python at the same time.

For the past 15+ years, Antonio Cuni has been a core developer of PyPy, a fast, compliant Python interpreter. Cuni notably co-founded HPy, a project providing a new API for extending Python in C.