Dredd is a command-line tool for validating API documentation written in API Blueprint format against its actual code. With Dredd you can easily plug your API documentation into a Continous Integration system like Travis CI or Jenkins and have API documentation up-to-date, all the time. Works to test API's written in PHP, Node.js, Ruby and Python. I've been using Swagger for API docs, but I'm looking forward to trying out Dredd + Blueprint in a future project.

I/O Docs is a live interactive documentation system for RESTful web APIs. Open sourced by the talented API guys at Mashery. I/O Docs is a lot like Swagger except I/O Docs isn't just a spec, it's the interpreter as well and it's written in NodeJS.

Swagger is a popular open source spec to auto-generate very nice looking API documentation with the ability to easily try any API request. You can use the Swagger's own swagger-core framework for node.js, Java or scala, but there's is also support in many other languages because of the strong open source community behind it for other languages like PHP, Ruby, Python, etc. Swagger's biggest open source competitor is Mashery's I/O Docs.