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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
My friend Tris has mentioned many times how positive his experience has been of the comments on his wildly popular Rust videos on YouTube.
I saw someone using the GitHub-based Utterances on their blog and thought it was pretty neat. I'm not sure who it was now, 'cos I can't find it. It was definitely someone in our Blog Guild - I thought it was Peter Makowski's blog but he doesn't seem to have it set up any more, so who knows. Anyway, Utterances looks neat.