diffmark is an XML diff and merge package.  It consists of a shared C++
library, libdiffmark, plus two programs wrapping the library into a
command-line interface: dm and dm-merge.  dm takes 2 XML files and
prints their diff (also an XML document) on its standard output.
dm-merge takes the first document passed to dm and its output and
produces the second document.

WWW: http://www.mangrove.cz/diffmark/