I've got several good ones bookmarked.

This page summarizes the projects mentioned and recommended in the original post on /r/ProgrammerHumor

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.
surveyjs.io
featured
InfluxDB - Power Real-Time Data Analytics at Scale
Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
www.influxdata.com
featured
  • RegExr

    RegExr is a HTML/JS based tool for creating, testing, and learning about Regular Expressions.

  • I always use http://regexr.com to build my regexes so that I can easily check that it’s doing what I want it to

  • magic-regexp

    A compiled-away, type-safe, readable RegExp alternative

  • 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.

    SurveyJS logo
  • DevToys

    A Swiss Army knife for developers.

  • Strictly speaking, DevToys isn't an online builder.

NOTE: The number of mentions on this list indicates mentions on common posts plus user suggested alternatives. Hence, a higher number means a more popular project.

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