Swagger UI

Swagger UI turns your OpenAPI spec into a live, interactive documentation page where anyone can try your API without writing code. Paste a spec file, get a browsable, testable doc site.

This is the standard. Apache 2.0, used by basically every team that documents APIs. You drop it into your project, point it at your OpenAPI JSON or YAML, and your docs are live. It renders every endpoint, shows request/response schemas, and lets users hit your API right from the browser.

The free version does everything most teams need. SmartBear (the company behind Swagger) sells SwaggerHub for hosted collaboration, versioning, and team management starting around $75/mo per user, but that's the platform, not the UI component. The open source UI itself has zero paywalls.

The catch: it only works with OpenAPI specs. If your API isn't documented in that format, you need to write the spec first (or use a code-first tool that generates it). And the default styling looks like 2018. Functional, not pretty. Redoc looks better out of the box if aesthetics matter to you.