xmllint - command line XML tool
XMLStarlet - XMLStarlet Command Line XML Toolkit
TXR - Pragmatic, convenient data munging language.
GNU sed - sed (stream editor) is a Unix utility that parses text and implements a programming language which...
XMLViewer - XML Viewer is a freeware for opening, editing, creating and saving XML files of any length.
GNU M4 - GNU M4 is an implementation of the m4 macro preprocessor.