Doxygen is a very cool documentation generator (HTML, LaTeX, RTF, PDF, etc) for C, C++ and IDL.
Free, portable, actively developed with a set of good mailing lists for support and developers.
See also: GraphViz, DocPlusPlus, JavaDoc, CppDoc
"Doxygen is a documentation system for C++, C, Java, IDL (Corba and Microsoft flavors) and to some extent PHP and C#. Doxygen can generate an on-line documentation browser (in HTML) and/or an off-line reference manual ... The documentation is extracted directly from the sources, which makes it much easier to keep the documentation consistent with the source code."
http://www.stack.nl/~dimitri/doxygen/
Doxygen is a BarelyLiterateProgramming system.
For ObjectiveCee, we have AutoDoc? and HeaderDoc?. The latter ships with ExCode on Panther (MacOsx 10.3). AutoDoc? has been around since NextStep/OpenStep.