Swagger / OpenAPI
Swagger UI is the canonical open-source renderer for interactive API documentation from OpenAPI specs, with 28k+ GitHub stars. SwaggerHub adds collaborative API design, versioning, and governance at scale. The de facto standard for REST API specification across the industry.
Redocly
Combines Redoc (25k+ GitHub stars) with a powerful CLI for linting, validating, and bundling OpenAPI specs. Supports OpenAPI 3.x, Swagger 2.0, AsyncAPI, and Arazzo. Enterprise offering adds developer portals and auto-generated code samples across teams.