Good API documentation improves the market adoption of APIs. The API documentation should be both human and machine readable. There are many tools available today for documenting RESTful web APIs. Some of the popular RESTful web API documentation tools are listed here for your quick reference:

• WADL: This tool is an XML description of HTTP-based web applications such as RESTful web services. Not many vendors use WADL nowadays due to the emergence of more developer-friendly API documentation tools such as Swagger, RAML, and API Blueprint. To learn more about the WADL specification, visit http://www.w3.org/Submission/wadl.
• RAML: This tool provides both human- and machine-readable formats (YAML) for describing APIs. This is relatively new in the market and is well supported by the active open source community. To learn more about RAML, visit the official site at http://raml.org.
• Swagger: This tool allows you to define APIs either in YAML or JSON and is backed up by a large open source community. More details can be found at http://swagger.io.
• API Blueprint: This tool provides a documentation-oriented web API description language that uses the markdown format for describing the APIs. It is backed by Apiary and a large open source community. To learn more, visit the official site at https://apiblueprint.org.

We discussed WADL, RAML, and Swagger in detail in Chapter 7, Description and Discovery of RESTful Web Services.