Command Line: XMLStarlet http://sourceforge.net/projects/xmlstar/files/xmlstarlet/1.6.1/
(A swiss army knife XML tool, packaged in a single standalone binary):
"XMLStarlet is a set of command line utilities (tools) which can be used to transform, query, validate, and edit XML documents and files using simple set of shell commands in similar way it is done for plain text files using UNIX grep, sed, awk, diff, patch, join, etc commands."
GUI: XML Marker http://symbolclick.com/
These guys used to sell their own XML parser and this program was the tech demo for the parser.
Claims to handle 500MB files in seconds.
(Has a nice way of viewing XML files.)