A documentation creation system that reads source code, extracts code structure and comments, and dumps out HTML/LaTeX documentation, complete with cross references and all bells and whistles.
It's Open Source™®©, so, if you're a programmer, you should bloody well use it, or you will probably regret it later when people beg for explanation of how your miraculous code works... or worse yet, you may later become curious. =)
DOC++ is related to Javadoc in functionality, but in addition to Java it also understands C/C++ and IDL.
The comment style is pretty straightforward and Javadoc users should feel just like home. Big comments go into
/** comment ... */, winged comments go into
/// comment..., and in addition to this some control codes are supported for some interestingness...