Open Source Internal Tool Development Platforms
2 open source internal tool builders you can self-host. Compare Retool, Appsmith, and Budibase alternatives by data sources, UI, and license. Updated Sept 2026.
ToolJet
Low-code platform for building internal tools and business apps
Windmill
Windmill is an open-source developer platform and workflow engine, and a self-hosted alternative to Retool, Airflow, and Temporal. Scripts written in Python, TypeScript, Go, Bash, or SQL become instant webhooks, scheduled jobs, multi-step flows, or auto-generated UIs. A Rust-based execution engine runs steps across distributed workers for high-throughput job processing.
Budibase
Budibase is an open-source low-code platform for internal tools, and a self-hosted alternative to Retool. You connect an existing SQL database, REST API, or spreadsheet — or use its built-in database — then assemble screens, forms, and automations from prebuilt components. Apps are deployed from the same tool, either to Budibase's own runtime or via Docker and Kubernetes.
Appsmith
An open-source low-code platform and a self-hostable alternative to Retool for building internal tools, admin panels, and dashboards. Appsmith connects to 25+ databases and any REST or GraphQL API, pairs a drag-and-drop UI builder with pre-built widgets, and lets developers add logic in JavaScript — with no per-user cost when self-hosted.