Halibut is a documentation production system, with elements similar to TeX, debiandoc-sgml, TeXinfo, and others. It is primarily targeted at people producing software manuals.
What does it do?

Halibut reads documentation source in a single input format, and produces multiple output formats containing the same text. The supported output formats are:

• Plain ASCII text
• HTML
• PDF
• PostScript
• Unix man pages
• Unix info, generated directly as .info files rather than .texi sources
• Windows HTML Help (.CHM files), generated directly without needing a separate help compiler.
• Windows WinHelp (old-style .HLP files), also generated directly.

Other notable features include:

• Hypertext cross-references are ubiquitous where possible. In particular, the HTML and PDF output both have hyperlinks in every reference between sections, and throughout the index and contents sections. (It seems daft to me that so many PDF documents fail to have this; it's one of the most useful features of PDF.)
• Comprehensive indexing support. Indexing is easy in the simple case: as you write the manual, you just wrap a word or two in \i{this wrapper}, and those words will appear in the index. * More complex indexing is also supported,