Tools for generating API documentation (similar to javadoc)
Good API documentation is an indispensible tool for any API or library.
The process of keeping that documentation in sync with the code is aided greatly by having an automated tool (in fact, it could be argued that in the absence of such a tool, it is unlikely to happen.)
The javadoc tool for java (wikipedia article) is one of the tools that is most familiar. For Java code, the javadoc tool can automatically generates documentation, by default, in the form of a website, for the public methods and properties of each class in a code base. It can do this even in the absence of any particular effort on the part of the code authors to document the code, but the usefulness of the generated documentation can be greatly enhanced by the addition of specially formatted comments at the top of each compilation unit (interface, class, method) that provide additional context, detail, and examples of the use of the object or method.
Similar tools exist for other programming languages. This page is a place to compile a list of those, along with pointers in information and useful tips.
This Stack Overflow question asks about tools for documenting node.js project.