Before a friend put a link to the product on their Facebook page, I had never heard of a whiskey disk. So, what is a whiskey disk?

It's a circular soapstone that you drop into your whiskey to keep it cool without having any dilution (like you would with ice). Why is it important not to dilute your whiskey? explains:

The degree of dilution directly affects the flavors of the spirit. It is a complex and weighty process; one best left to the masters of the trade. Unless you purchase cask-strength, you have no real need to add water. Most bottled whiskies are already diluted with water to bring their strength down to a level that makes it less expensive to produce and more palatable, usually about 40% alcohol by volume. So, the two things you don't want to do are take temperature below a certain point and dilute unintentionally. This is where adding ice becomes dicey, because it is not a very precise way to either chill or add water to spirits. Too much ice can destroy the complex flavor structure due to temperature or dilution.

I haven't seen these in town at any clubs or restaurants yet, but I'm sure they will be popping up soon. You can order them for home use; a set of four costs about $30.