Duel Masters

CCG Database: Duel Masters
Offers an extensive card search including both English releases and Japanese translations.
http://www.ccgdb.com/duelmasters/

Duel Masters: Trading Card Game
Official site from Wizards of the Coast. Offers product information, general series information, a card database, events, rules, and community.
http://www.duelmasters.com/

Duel Zone
Offers forums, chat, rules, and the Duel Masters Civilizations online play program.
http://www.duelzone.org/

Wikipedia: Duel Masters
Encyclopedia article which describes the plot, characters, civilizations and notes the differences between the Japanese and American versions.
http://en.wikipedia.org/wiki/Duel_Masters