One is Splinter Twin with Deceiver Exarch. This caused a bump in the price of Splinter Twin sending it from $3 to around $15.
The combo is simple, you play a Deceiver Exarch then enchant it with Splinter Twin, you then tap the Deceiver Exarch to get a token of it. Deceiver Exarch's text states
When Deceiver Exarch enters the battlefield, choose one - Untap target permanent you control; or tap target permanent an opponent controls.So you chose to untap your Deceiver Exarch enchanted with Splinter Twin then tap it again to create another token, by turn 4 you're able to create an infinite amount of 1/4 creatures, all with haste so they can attack that turn.
Another intresting combo, but one that takes some building up, is Bloodchief Ascension with Mindcrank. Mindcrank causes the opponent to mill cards whenever they take damage, Bloodchief Ascension causes the opponent to take damage whenever a card goes into their graveyard.
The result? Infinite damage and infinite milling since they both cause each other to trigger, you also gain life but that hardly matters when you've won.
It's a nice combo but for Bloodchief Ascension to work, they need to have lost 2 or more life a turn for 3 turns. But in a multiplayer game, since they don't target, you just have to have 3 opponents lose 2 or more life, then once any of them loses a life or puts a card into their graveyard, they would lose the game.