Natural Docs is an open-source documentation generator for multiple programming languages. You document your code in a natural syntax that reads like plain English. Natural Docs then scans your code and builds high-quality HTML documentation from it.
Natural Docs is trivial to install, and it's very easy and quick to get started. The natural syntax used by Natural Docs is so natural it's almost possible that you could accidently have comments in your code already in the required syntax :-)