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.
Stoplight
Design-first platform for creating, governing, and documenting OpenAPI specifications. Bundles a visual editor, Spectral (JSON/YAML linter), Prism (mock server), and Elements (docs renderer) into one workflow. Teams use it to enforce API style guides across large API portfolios.