This one's fun, easy, and best of all, can be done in your own kitchen (plus, you'll actually "fool" yourself). I'm not really sure how to use it to score a drink at the bar, but damn does it look cool.
Put a few unopened bottles of beer in your freezer for a couple of hours (side note: the visual effect will work best with clear beer bottles, like Corona). When it comes time to remove the beers, you may notice a couple of them have frozen and "exploded", but there should also be a few that are still in a liquid state. Carefully pick these up, set them on the table, and remove the bottle caps.
At this point, the trick does itself... just give the bottle a good tap on the side of the counter, and as the bubbles start to form, the entire beer will instantly freeze, eventually pushing the ever-expanding ice out the mouth of the bottle.
Why does this work? In a word: supercooling. From Wikipedia:
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid.
A liquid below its standard freezing point will crystallise in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. The homogeneous nucleation can occur above the glass transition where the system is amorphous (non-crystalline) solid.
In the case of our beers, when we strike the bottle, the gas bubbles that form immediately give the supercooled liquid a nucleus around which to freeze. It is possible to supercool water bottles in the same manner, but I've found it much easier with glass bottles of beer than plastic bottles of Diet Coke.
Remember that these are glass bottles filled with a supercooled liquid and that water expands as it freezes to ice. I've never seen one of these violently explode, but I suppose it's possible. Make sure you take appropriate safety precautions.
I'd mention something about kids not trying this, but then I just think "What kind of idiot gives a bunch of bottles of beer to kids for them to play with?"