xmllint - command line XML tool
GCC C Preprocessor (cpp) - Top (The C Preprocessor)
Xidel - Xidel is a command line tool to download html/xml pages and extract data from them using CSS 3 selectors, XPath 3 expressions or pattern-matching templates.
Filepp - filepp is a generic file preprocessor.
PP - A generic Preprocessor - P is a text preprocessor designed for Pandoc (and more generally Markdown and reStructuredText).
Gema - General purpose text macro processor.