OFFICIAL Shonen Jump Website (in Japanese) www.Jump.ne.jp
OFFICIAL Toei Animation Studio Yuughioh Website (in Japanese) www.Toei-anim.com.jp
Fan Sites
Yuugi's Playhouse the most definitive English guide to Yuugioh on the net. This site has info on the manga series, both anime series, the card game Duel Monsters (including English translated instructions for the cards).
YBFREE.com