DocBook and the tools we use to work with it are Open Source.

LaTeX is nice for mathematical markup, but DocBook exists for marking-up technical documentation.

Basic HTML is easy to learn and use, but it is very clumsy. DocBook does a good job of separating content from presentation (HTML does not do this particularly well, even with the use of CSS), allowing the writer to focus on what really matters, the content.

DocBook can transformed into high-quality online and printed output (e.g. HTML and PDF, respectively), while working from a single source.

DocBook is very robust, thoroughly documented, and has a strong community behind it.

DocBook is a modern though mature standard for documentation of software (or other) projects which is at no risk of obsolescence.

The tools needed for creating and manipulating DocBook documents are typically part of existing *nix installations (including Cygwin) so little or no installation of new software is required to use DocBook (editing can be done in any text editor).

Many tedious tasks, such as the creation of a table of contents, are handled automatically by a good DocBook configuration.