Well, here is a list of cards that made me win, usually getting my opponent to concede, even if I intended to use other stuff for victory.
Some players just hate a card so much they'll leave.
Thieves' Auction 
is never intended for a direct win in itself, but usually losing their best creatures and some of their mana base, makes the other player(s) leave. I say it from experience.
Since not everybody plays cards that destroy artifacts,
Crawlspace 
made me win a few times, combined with
Serra's Blessing 
in a angels deck.
Surprisingly,
Feldon's Cane 
won me a game in wich my opponent made everybody draw and discard and since I also had a
Howling Mine 
on the field (he did have 1 too),
turns out he emptied his own library.
Apocalypse 
made me stop all what was going on in a few games (I never won but got a lot of hatred for playing it twice in a game, same for
Thieves' Auction ) and I was lucky to draw enough mana to cast my burn spells to seal the win after that.
Steel Golem 
given by
Bazaar Trader 
led to some conceding. That trader is great to give gifts to your opponents.
In a 2HG, where I had more of a
support blue deck, I didn't have much defense and for that I was getting beated pretty badly, my partner was partially mana screwed and so was I. Finally. 2 turns in a row I got the islands I needed to cast
Acquire 
on the opposing head with an artifact deck.
Well, I found
Platinum Emperion 
(
Blightsteel Colossus 
and his set wasn't released yet), and since my parter got mana too during those 2 turns he was able to cast & kick
Rite of Replication 
on the Emperion, and that finally made us able to beat the other giant.
