Doxygen

Doxygen is a documentation system for C++, Java, IDL (Corba, Microsoft and KDE-DCOP flavors) and C.

It can help you in three ways:

1. It can generate an on-line documentation browser (in HTML) and/or an off-line reference manual (in LATEX ) from a set of documented source files. There is also support for generating output in RTF (MS-Word), PostScript, hyperlinked PDF, compressed HTML, and Unix man pages. The documentation is extracted directly from the sources, which makes it much easier to keep the documentation consistent with the source code.
2. Doxygen can be configured to extract the code structure from undocumented source files. This can be very useful to quickly find your way in large source distributions. The relations between the various elements are be visualized by means of include dependency graphs, inheritance diagrams, and collaboration diagrams, which are all generated automatically.
3. You can even `abuse' doxygen for creating normal documentation (as I did for this manual).