Please check out Swagger Codegen (free, open-source), which can generate both server stubs and API clients in different languages.
Many companies/projects are using it in production: https://github.com/swagger-api/swagger-codegen#companiesprojects-using-swagger-codegen
Supported langauges/frameworks:
API clients: ActionScript, Bash, C# (.net 2.0, 4.0 or later), C++ (cpprest, Qt5, Tizen), Clojure, Dart, Elixir, Go, Groovy, Haskell, Java (Jersey1.x, Jersey2.x, OkHttp, Retrofit1.x, Retrofit2.x, Feign), Node.js (ES5, ES6, AngularJS with Google Closure Compiler annotations) Objective-C, Perl, PHP, Python, Ruby, Scala, Swift (2.x, 3.x), Typescript (Angular1.x, Angular2.x, Fetch, jQuery, Node)
Server stubs: C# (ASP.NET Core, NancyFx), Erlang, Go, Haskell, Java (MSF4J, Spring, Undertow, JAX-RS: CDI, CXF, Inflector, RestEasy), PHP (Lumen, Slim, Silex, Zend Expressive), Python (Flask), NodeJS, Ruby (Sinatra, Rails5), Scala (Finch, Scalatra)
API documentation generators: HTML, Confluence Wiki
Disclaimer: I'm one of the top contributors to the open-source project.
UPDATE: On May 2018, about 50 top contributors and template creators of Swagger Codegen decided to fork Swagger Codegen to maintain a community-driven version called OpenAPI Generator. Please refer to the Q&A for more information.