Yu-Gi-Oh

Gamers-TCG
Offering singles, promos, Chinese versions, tins, boosters. Features a tournament section.
http://www.gamerstcg.com/

YuGiOh Card Stop
Carries Konami Japanese and English Upper Deck cards, packs, decks and boxes.
http://www.yugiohcardstop.com/

Yugioh-cards.net
Featuring boxes, booster packs and hard to find single cards. Contains rules and instructions for the game.
http://www.yu-gi-oh-cards.net

Yu-gyo.com
Offering promo cards with translations for collectors and players.
http://www.yu-gyo.com/