Good old perlite! It's been around for years, mainly for use as a soil additive to increase aeration and draining of the soil. Perlite is a mined material, a form of volcanic glass that when rapidly heated to more than 1600 deg. f. it pops much like popcorn as the water vaporizes and makes countless tiny bubbles.
Perlite is one of the best hydroponic growing mediums around. Used by itself or as a mixture with other mediums. Perlite is commonly used with vermiculite ( a 50 - 50 mix is a very popular medium), and is also one of the major ingredients of soiless mix's.
Perlite has good wicking action which makes it a good choice for wick-type hydroponic systems. Perlite is also relatively inexpensive. The biggest drawback to perlite is that it doesn't retain water well which means that it will dry out quickly between waterings.
The dust from perlite is bad for your health so you should wear a dust mask when handling it.