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pencil
The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
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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.
I am a software developer, so doing UX is never my strength. From time to time though, I would resort to using the open source tool Pencil (https://pencil.evolus.vn/) to get a low fidelity mock-up. Lately I've been encountering bugs where images would come out broken when I re-open my wireframe in Pencil.
Frustrated of the issue on Pencil, I tried out uizard yesterday, and have been really happy with it. It's almost as convenient and fancy as Figma, yet it's much more easy to use. I was able to pull up a decent looking wireframe in a relatively short amount of time.
Just sharing my experience here as a happy user, cos somehow I'm not seeing that many articles about uizard here. :)