Streamlit

Streamlit turns Python scripts into web apps in minutes: dashboards, demos, internal tools. No HTML, no CSS, no JavaScript. You write Python, and Streamlit gives you a web interface with charts, tables, sliders, and file uploads.

Apache 2.0, Python, owned by Snowflake. The workflow is dead simple: write a Python script using Streamlit's API (`st.write`, `st.chart`, `st.slider`), run `streamlit run app.py`, and you have a web app. It re-runs the script top-to-bottom on every interaction, which is brilliant for prototyping and awkward for complex state management.

The library is fully free. Streamlit Community Cloud hosts your apps for free with generous limits: public apps, GitHub-connected deployment. Snowflake offers Streamlit in Snowflake for enterprise data apps. Solo data scientists: this is your tool. Build demos, share analyses, prototype ML interfaces. Small teams: great for internal dashboards. Medium to large: works for internal tools, but production customer-facing apps will outgrow the re-run-on-every-click model.

The catch: Streamlit re-executes your entire script on every user interaction. For simple apps, fine. For apps with expensive computations, you need caching decorators everywhere. The layout system is limited: you get a single column by default and basic column/tab options. If you need a real web app with complex navigation and state, you've outgrown Streamlit.