The Millennium Key in fan-fics.
Yu-Gi-Oh! X - The Millennium Key was recharged by the soul of the curator who awoke the evil in Pharaoh Seto's tomb. Shadi then used the Millennium Key to see if Tag was mentally worthy enough to bear a Millennium Item. He then used the Key to search Bakura for signs of guilt, and when he discovered that Tag and the gang were coming, he turned Bakura into a mindless slave. As Bakura chased off Yugi and Johnny, Shadi used the Key to turn the girls into dolls. The Key was then held up by Shadi's Ushebti, so if Tag, Seth, and Darin managed to shock Shadi enough to break his Ushebti, the Key would travel down a rope and bump against the girls hands to turn them back to normal, and that happened when Shadi was shocked by Darin's renewed trust in his mother.
Yugi Moto and the Duelist's Stone - The Millennium Key is used to sort new students at Hogwarts into the school houses. The Key has a mind of its own as it seriously considered putting Yugi into Slytherin before he begged not to be with his mind.