MediaInfo is a great all-around video file tool.
http://mediainfo.sourceforge.net/en
MPEG Parser by DVD Logic is good (Windows)
http://www.dvd-logic.com/mpegparser.htm
Trial version of CodecVisa (Windows) will display GOP structure
Full version is expensive.
http://www.codecian.com/features.html
Free software MPEG-2 Transport Stream packet analyser (Windows)
http://www.pjdaniel.org.uk/mpeg/index.php
