Duel winners are a series of cards capable of automatically winning a Duel once certain unique conditions have been met. They are distinct from Match winners, in that these cards only help to win a single Duel.
The following is a list of current Duel winners in the OCG or TCG:
"Last Turn": When you activate this card by having 1000 or fewer Life Points, you choose one monster you control, and all other cards on the field and in both players' hands are sent to the Graveyard. Your opponent then Special Summons a monster from their Deck. A special Battle Phase occurs in which the two monsters battle and Battle Damage is ignored. The player whose monster remains alone on the field at the End Phase of this turn wins the Duel. This card is currently Forbidden.
"Number 88: Gimmick Puppet of Leo": This card can gain a Destiny Counter once per turn by detaching one of its three Xyz Materials, but in order to do so, you must have no cards in your Spell & Trap Card Zone and must skip that turn's Battle Phase. If this card has three Destiny Counters on it, you win the Duel. (In the anime, this card could detach one of its Xyz Materials once per turn with no restrictions, and it allowed its controller to win the Duel if it detached all of its Xyz Materials by its own effect, with no Counters involved.)
"Number C88: Gimmick Puppet Disaster Leo": Once per turn, this card can detach an Xyz Material to inflict 1000 damage to the opponent. During your End Phase, if this card has no Xyz Materials while the opponent has 2000 or fewer Life Points, you win the Duel. (In the anime, the damage was 4000 instead of 1000, and it allowed its controller to win the Duel if it had no Xyz Materials at the end of a turn, with no requirement on the opponent's Life Points.)
"Jackpot 7": This card banishes itself when sent to the Graveyard by your opponent's card effect. When three "Jackpot 7" are banished by this effect, you win the Duel.
"Relay Soul": This card allows you to Special Summon a monster from your hand, but if it leaves the field, then your opponent wins the Duel. (In the anime, this card activated when the user's Life Points became 0 and prevented the user from losing, instead making the opponent's only victory condition the destruction of the Special Summoned monster.)