GPP - GPP is a general-purpose preprocessor with customizable syntax, suitable for a wide range of...
GNU M4 - GNU M4 is an implementation of the m4 macro preprocessor.
xmllint - command line XML tool
Python - Python is a clear and powerful object-oriented programming language, comparable to Perl, Ruby, Scheme, or Java.
PP - A generic Preprocessor - P is a text preprocessor designed for Pandoc (and more generally Markdown and reStructuredText).
Filepp - filepp is a generic file preprocessor.