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". Changes not directly affecting visible text, for example in formatting commands, are still marked in the latex source.
A rudimentary revision facilility is provided by another Perl script, latexrevise, which accepts or rejects all changes. Manual editing of the difference file can be used to override this default behaviour and accept or reject selected changes only.
The author is F Tilmann.