API's (Application Program Interface) are a set of routines that are used to interact with web applications. API's provide a secure and easy way for applications to open up to 3rd party developers. Analyzo has listed a set of tools that help applications build web API's.
Apiary makes API development simple and collaborative. The Apiary platform is the only developer-centric solution with tooling for every stage of the API lifecycle: designing, prototyping, testing, and documentation.
Apigility is an API Builder, designed to simplify creating and maintaining useful, easy to consume, and well structured APIs. Regardless of your experience in API building, with Apigility you can build APIs that enable mobile apps, developer communities, and any other consumer controlled access to your applications.
Falcon is a fast, minimalist Python web framework for building cloud APIs and app backends. The Falcon web framework encourages the REST architectural style, meaning that you think in terms of resources and state transitions, which map to HTTP verbs. Falcon is an Apache-licensed Rackspace community project, built and maintained by a group of volunteers from around the world.
LoopBack is a highly-extensible, open-source Node.js framework that lets you quickly create dynamic end-to-end REST APIs. It lets you use Android, iOS, and AngularJS SDKs to easily create client apps. Also the user can add-on components for push, file management, 3rd-party login, and geolocation.
SwaggerHub is an integrated API design and documentation platform, built for teams to drive consistency and discipline across the API development workflow. It is built by the creators of the popular Swagger open source tooling, and allows teams to develop APIs faster on a centralized platform.