Ask HN: Are open-source websites possible/practical?

Suppose one has an idea for a web service that is universally beneficial and will make the world a better place. There is lots that can be done and myriad directions to go in, but it should be completely free (modulo hosting costs, that can be covered) and best run by volunteers. For regular code open-source is great, but how about extending this to the full design and development of a website. Design, front-end, back-end all submitted by regular people. Is this plausible, and how would you see it happening? Would it be better to use Rails or Django or what? Either way anyone could fork/clone the project and run it for free on Heroku/AppEngine (with test data) and then submit changes to the main project.