Is there an experiment that visibly shows the effects of metastability? I am attempting to create an experiment to show the effects of metastability to a public audience and get people excited about science. More specifically, I am wondering if there is any sort of liquid that can mimic the effects that water had in the book Cat's Cradle. 

In the book, the idea was that a shard of a more stable polymorphic crystal of ice with a high melting point could "infect" water. This crystal would quickly spread, solidifying oceans and destroying most life on earth. 

The basic question I'm asking is as follows. Is there any public display I could make to show a liquid quickly solidifying from a seed crystal? It is preferable (but not necessary) for the materials to be reasonably inexpensive and require no special equipment, so as to allow for the experiment to be done on a grander scale. Additionally, the experiment should have a low failure rate (unlike using supercooled water, which is difficult to control in large amounts).
The standard demo for this is crystallization of sodium acetate trihydrate from its melt.  The trihydrate melts at 58 &deg;C.  The molten liquid is easily supercooled, after which point dumping in a few crystals of unmelted stuff results in a rapid solidification of the mixture.

Several videos on YouTube demonstrate the reaction; Nurdrage's is my personal favorite.