I don't know if this will go into the pantheon with 47...Bh3!! and moves of that ilk, but the coup de grace administered by Peter Svidler against Gata Kamsky is at least a contender, especially considering that he did it against a great player in an important event. Here's the situation: White to move has just played 26.Nc6xb8:
Perhaps Kamsky expected 26...Qxh6, when after 27.Nc6 Kg7! Black has sufficient compensation for the exchange, but not more. And no doubt he was ready for 26...Qg3, when 27.Nc6 would win, were it not for a strong move that equalizes. It turns out that that same strong move is even better when played immediately: 26...Re2!!
Fantastic! Black is down a rook, and puts a second rook en prise on an empty square. Of course, its virtue is clear if White doesn't take the rook - Black crashes through on f2. The crucial point is that if White does take the rook, Black plays 27...Qg3 and White is helpless against the threat of ...Qxg2#. When the queen was on c2, Nc6 was an adequate reply to ...Qg3, but now Nc6 is just a spite move.
So Kamsky played 27.Qc3, and resigned after 27...Rxf2 28.Nc6 Rxf1+, as 29.Kxf1 Qf2 is mate and 29.Kh2 Rxa1 gives Black an extra rook and pawn with mate on the way.
That gave Svidler a 2-0 victory in the match, and he will play the winner of tomorrow's tiebreaker between Judit Polgar and Leinier Dominguez. Dominguez won game 1 yesterday and had a very promising position today, but at a crucial moment made the wrong choice and suffered for a long time. It took Polgar 112 moves, and she made at least one crucial error and a large number of inaccuracies in converting her advantage, but she kept plugging away to win the game.
She was not alone in fighting back from a defeat in the first game - Peter Heine Nielsen did it as well, grinding out a victory against Vugar Gashimov in 113 moves! (The game went one move longer but a few seconds shorter than Polgar's victory.) A third comeback hero was Alexander Grischuk, who finally stopped the Vladimir Potkin steamroller with some nice tactics on the white side of a Classical French.
In addition to those matches, two others will go to tiebreaks; namely, the two that saw draws in yesterday's games. Ponomariov-Bruzon was a "correct" draw, while Ivanchuk-Bu Xiangzhi had some ups and downs before the point was split after 95 moves. (It's some round when that's only the third-longest game.)
Finally, in addition to Svidler, two other players assured themselves of a rest day tomorrow. David Navara was always doing fine against Yaroslav Zherebukh, but fine turned into winning after the latter blundered with 28...Rf6?? Still, despite losing the match 2-0, the 18-year-old Zherebukh accounted himself extremely well in this tournament, and one would expect to hear a lot more from him in the next few years to come.
Finally, Jakovenko-Radjabov was a short draw, the only one today (and only the third draw in eight games and fifth in the sixteen games played in the round overall!), and that sent Radjabov through to the quarterfinals. It's not that Jakovenko pulled a Morozevich and gave up without an effort, but that he was, if anything, a bit worse in the final position and without any real winning prospects (though he could have continued a bit anyway).
So at this point none of the quarterfinal matches are set. Here's a recap: Svidler won his match and is waiting for the winner of Dominguez-Polgar. The winner of Ponomariov-Bruzon will play the winner of Gashimov-Nielsen. Radjabov will play the winner of Ivanchuk-Bu Xiangzhi, and Navara will play the winner of Grischuk-Potkin.
The official site is here, and today's games (with my comments) are here. Note: I will cover the Kamsky-Svidler game in much greater depth for my ChessVideos show this week (those are free and available to the general public, for those of you who might be new to the site), which should be posted online tomorrow or Friday. (Fear not, there will be an announcement on this site once it's up.)