If you are talking about a web(dav) folder on a webserver, I've used TinyAuth with good effect as a minimalistic access control.

My plan to leverage the idea of TinyAuth and to combine it with OAUTH 2 so that people can "log in with Facebook" (etc.) failed because one would have to register every instance of the TinyAuth application with Facebook and all the other suppliers one would want to support. I did not find a convenient way to automate registering an application with all those providers.