filepp is a generic file preprocessor designed to allow the functionality provided by the C preprocessor to be used with any file type. It supports the full set of C preprocessor keywords (#include, #define, #if, etc.). filepp is also highly customisable and allows users to easily add their own keywords or modify the behaviour of existing keywords.
GNU TeXmacs is a free wysiwyw (what you see is what you want) editing platform with special features for scientists. The software aims to provide a unified and user friendly framework for editing structured documents with different types of content: text, mathematics, graphics, interactive content. TeXmacs can also be used as an interface to many external systems for computer algebra, numerical analysis, and statistics. New presentation styles can be written by the user and new features can be added to the editor using Scheme.
The Guava tools are a set of Perl scripts for HTML pre-processing. You can create multi-page documents with contents tables, or use templates to give a consistent look to a set of pages. All output is passed through the C preprocessor, so you can use directives such as #include, #define and #if. There are also built-in macros for producing dates, cross references, etc.
HTML PLAIN is a "HTML Preprocessor Lacking An Interesting Name". Symbolic names instead of filenames allows files to be moved around without breaking links. Templates allow easy customization and changes of the layout. Tags can be redefined for a better structure of the document. Beginners can use one of the given templates and do not have to use any markup language at all. Experts can write their own Perl macros in order to get access to metadata and more flexibility.