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.
Stainless
Accepts an OpenAPI spec and produces high-quality, idiomatic SDKs that feel hand-written across TypeScript, Python, Go, Java, Kotlin, Ruby, PHP, and C#. GitHub integration auto-regenerates SDKs when the spec changes. The team behind OpenAI, Anthropic, Cloudflare, and Meta's official SDKs.