If you write API documentation, a feature-rich tool is what you need. Here, I gathered some great solutions.

Swagger

Swagger is a powerful yet easy-to-use suite of API developer tools for teams and individuals, enabling development across the entire API lifecycle, from design and documentation, to test and deployment.

Swagger consists of a mix of open-source, free and commercially available tools that allow anyone, from technical engineers to street smart product managers to build amazing APIs that everyone loves.

• Design. Design and model APIs according to specification-based standards.
• Build. Build stable, reusable code for your API in almost any language.
• Document. Improve developer experience with interactive API documentation.
• Test. Perform simple functional tests on your APIs without overhead.

ClickHelp

ClickHelp is a modern browser-based documentation tool used by software companies all over the world to create online user manuals, knowledge bases, help files, FAQs, tutorials and publish them instantly in their portal.

ClickHelp is easy to set up and use — no installation, runs in a web browser and supports multiple platforms: Mac OS, Windows, and Linux. This makes it easy to author your documentation from any place in the world.

This documentation tool has all the powerful features you may need for larger projects:

• Accessibility. ClickHelp is accessible according to Section 508 and WCAG 2.0 requirements.
• Password-protected online help.
• Easy importing and exporting. ClickHelp can import content from Microsoft Word, HTML, RTF, CHM, ODT, etc. and export to CHM, HTML5 Web Help, PDF, DOCX, etc. So, if you want to move from another tool, you can import your documents and that’s all.
• Powerful Full-Text Search. ClickHelp has its own patented full-text search that helps you and your readers find topics easily, it supports wildcard, and it’s also possible to exclude specific topics from full-text search results. A popular search result can be shown as a featured snippet to provide info faster.
• Reporting. ClickHelp has internal analytics that will help you measure team performance metrics and analyze end-user behavior statistics.
• Context Help engine to add context help elements to your web applications.
• Powerful branding options. You don’t need to know CSS to design your documentation, ClickHelp provides 6 premade modern templates. If you need more branding settings, you can also customize every part of the reader UI using CSS.
• Easy to use WYSIWYG topic editor, as well as HTML source mode.
• and much more!

API Doc

Hosted public API documentation for every OAS (Swagger) and RAML spec out there.

Here is a little FAQ:

How much does API Docs cost?

It’s completely free, forever! We believe that every API should have accessible, user friendly documentation. Features like custom domains, themes, and analytics, are available for a nominal cost through the StopLight integration.

Which specification formats are supported?

Currently, we support OAI (Swagger 2) and RAML 0.8. If your API is described in another format, like Swagger 1 or API Blueprint, we recommend converting to OAI first.

Can I edit my docs?

Not directly with api-docs.io. The StopLight API designer integrates tightly with API Docs, and includes many tools to edit API models/definitions.

Can I use my own custom domain? Yes

DapperDox

DapperDox is Open-Source, and provides rich, out-of-the-box, rendering of your OpenAPI specifications, seamlessly combined with your GitHub Flavoured Markdown documentation, guides and diagrams.

DapperDox was created by API documentation authors for one purpose: to improve the quality and usability of the API documentation they were providing for other developers.

They needed the flexibility to structure and style the pages the way they wanted; to be able to author readable guides and have them form part of a cohesive set of documentation along with the API specifications; to allow relevant documentation to be injected right into the rendered specification page, just where it was needed.

Readme.io

ReadMe gives teams the tools they need to create and manage beautiful documentation with ease, monitor their APIs, and connect with their users in more personal ways. It comes with a Markdown-based editor, versioning support, an API explorer to demonstrate sample calls, and crowdsource features where users can suggest API changes.

• Build documentation that’s interactive, personalized to each developer, and aligned with your brand.
• Collaborate with your developers, notify them about changes, and quickly answer their questions.
• Get insights into how developers are using your API, and prevent most problems before they happen.