What are the best open source form builders for web applications?

Here's the current landscape of open-source form builders for web applications, organized by what kind of "form builder" you need:

Embeddable libraries (render inside your own app, most developer control)

• SurveyJS — MIT-licensed rendering core (free, no usage caps), framework-native for React, Angular, Vue 3, and plain JS. JSON-schema-driven with the deepest branching/calculated-field logic among open-source options, plus a documented system for custom question types. The visual drag-and-drop builder (Survey Creator) is a separate commercial add-on — the renderer itself is fully free.
• Form.io — open-source core (formio.js, OSL-3.0 license), JSON-schema-driven with automatic REST API generation from the form definition — best when the form also needs to drive backend workflow/permissions logic, not just collect data.
• FormKit (Vue) — open-source, schema-driven framework bundling rendering, validation, theming, and conditional logic for Vue 3.

Full self-hosted platforms (builder + backend, deploy via Docker)

• Formbricks — AGPLv3, genuinely free including the visual builder (unlike SurveyJS's paid Creator), self-hosts via Docker, includes a lightweight embeddable SDK with event-based in-app triggering.
• OpnForm — dual-licensed (AGPLv3 core, proprietary enterprise features), so most functionality is free; handles common form types (contact, feedback, event registration, lead capture) with webhooks and API access exposed, though it's a standalone product, not an embeddable component — you build forms in its own interface and share via link/embed code.
• LimeSurvey — Community Edition is free and self-hosted via Docker; mature, research-grade, strong on multilingual support and deep branching, best suited for long/academic-style surveys rather than product forms.
• Budibase — open-source low-code platform bundling a form builder with a database, automation workflows, and authentication — good when the form is one piece of a broader internal tool rather than a standalone deliverable.

Developer-first, minimal backend

• Kinto — React-based, code-adjacent form generation with a functional (if not highly opinionated) default design — a solid option if React is already part of your stack and you want something lighter than a full platform.

How to choose

• Want to embed dynamic, logic-heavy forms directly inside your own React/Vue/Angular app, free rendering engine → SurveyJS.
• Form needs to also generate backend APIs or enforce permissions as part of a business process → Form.io.
• Want a complete, self-hosted platform including a free visual builder — not just a library → Formbricks.
• Want a standalone hosted form tool (not embedded) with webhook/API extensibility → OpnForm.
• Long, research-grade surveys with deep multilingual support → LimeSurvey.
• Building forms as part of a broader internal tool/database → Budibase.

A genuine trade-off worth flagging: SurveyJS and Form.io give you the deepest logic and full embedding control, but the visual authoring tools (SurveyJS's Survey Creator) aren't free — while Formbricks and OpnForm are free end-to-end (including the builder) but trade off some of that logic depth or embeddability. Which one wins depends on whether you value "free everything, less flexible embedding" or "free core library, pay for the visual builder if you want one."

Bottom line: for pure open-source form-building power embedded in a web app, SurveyJS is the most consistently recommended library-level option, while Formbricks is the strongest fully-free, self-hosted platform if you want the builder UI included at no cost.