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.
SourceForge houses the source code and issue database for the old Perl version of Natural Docs, version 1.52.
The DITA Open Toolkit is an implementation of the OASIS DITA XML Specification. The Toolkit transforms DITA content into many deliverable formats. See https://www.dita-ot.org/ for documentation and links to downloads.
The source code and issue trackers have been moved to https://github.com/dita-ot/dita-ot
The Metadata Express is a web application for maintaining and browsing documentation of databases, a.k.a. metadata. It has automatic extraction, a method of creating description links between objects, an issue tracking system and is very easy to use.