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Criss Angels' Walking On Water...........

sinful is one of the best impromtu tricks i know

I'm not an owner of this myself, but from what I have heard: self-tying shoelace. It's not entirely impromptu, but you can make it appear that way.

If by impromptu, you mean not needing much prior preparation, then Heirloom, the BIP book, coin unique, watch stopping, pulse stopping, cig pull...all these things can be done at a seconds notice and astonish the layman.
And of course, a deck of cards and a study of the RRTCM means you have a vast amount of impromptu effects at the tip of your fingers.