Playing 2/5NL in North Florida - two older fish had been battling it out all night and both had over 1k behind. There was heavy betting on a flop and turn of K-10-7-3, with three hearts on the board. The river brings the 10

, first to act bets out $150 and the older Mexican fellow thinks for a second, stares down his opponent, flips over the A

and announces he's all in for another $700 or so. Of course the guy snap calls and flips over quad 10's.
I loved this play so much because the thinking behind it was just absolutely mind-numbing. By showing the A

he ensured that nothing worse than his hand would ever call - only better. And just to make it clear, he wasn't trying to be nice to his opponent, he actually had some choice words for him as he left the poker room.