Dice stacking is the art of scooping up dice in a dice cup and setting the cup down to build a vertical column of dice. Most people's initial reaction, when first seeing the dice stacked, is astonishment.
Update 11/15/06 by Alex: Yayo's post inspired me to look for more dice stacking clips on YouTube. Turns out there's a lot:YouTube version of the devilducky link above:
A play-by-play demo:
Here's Brad Manuel stacking 25 dice: