This has been around for a couple of years now and actively maintained and supported, and is in use by many organisations already. It supports all versions of Excel from 2000 onwards, with some features only available in later versions (eg asynchronous functions are only in Excel 2010).
It uses cPython so all the standard libraries are usable.
Calling back into Excel is quite straightforward using win32com, and there are examples on the website that show how to do that.
With PyXLL you can write plain Python code that you can use outside Excel as well - so it's quite easy to write a library of functions that can be called from normal Python scripts as well as expose them to Excel at the same time.
I'm not sure what this project adds over what's already available but I'd be very interested to hear what other people's opinons are.