Postman is the complete toolchain for API developers, used by more than 3 million developers and 30,000 companies worldwide.
Swagger is a powerful open source framework backed by a large ecosystem of tools that helps you design, build, document, and consume your RESTful APIs.
The Apiary platform is the only developer-centric solution with tooling for every stage of the API ... Big news for #api developers blog.apiary.io/2016/01/18/W
Sphinx is a tool that makes it easy to create intelligent and beautiful documentation, written by Georg Brandl and licensed under the BSD license.
A tool for creating and maintaining excellent Developer Portals and API or Technical Documentation