RefDB is a reference database and bibliography tool for SGML, XML, and LaTeX documents, sort of a Reference Manager or BibTeX for markup languages. It is portable and known to run on Linux, Free/NetBSD, OSX, Solaris, and Windows/Cygwin.
latexdiff is a Perl script, which compares two latex files and marks up significant differences between them (i.e. a diff for latex files). Various options are available for visual markup using standard latex packages such as "color.sty".
otl is a text processor for generating markup from plain text. Much of both the input and output formats can be customized. otl supports structures such as nested ordered lists, headers and footers, and tables.