Shameless plug: I'm currently building a web app[0] that attempts to digitize the notecard format in a way that makes note-taking easy and powerful. It's in its infancy, but I would like to integrate better support for some zettelkästen methodologies down the line. The "linking" in our platform isn't quite as powerful as zettelkästen for some use-cases, but we do utilize a "parent-child" method of organizing your notes hierarchically.