Is there a depository/repository of Mathematica notebooks (not necessarily Mathematica packages) where one can upload some of their notebooks so as others can access them easily?
Online collaboration using a notebook interface is one of the many use-cases that the Wolfram Cloud was specifically built to address. See https://mathematica.wolframcloud.com/app/, and click around to explore. I'm not sure what level, if any, of subscription is needed in order to share saved notebooks, or whether you are able to make such notebooks public versus granting permissions to a pre-defined set of collaborators, but by poking around, I'm sure one could find these answers. Good luck!