Coco Fiber
Coco Fiber is an extremely popular hydroponic growing media and for good reason. Not only is coco fiber (which comes from coconut husks) a totally sustainable product, but it also has a naturally ideal pH range (5.8-6.3) for healthy plant growth. In addition to this, coco fiber has a great air to water ratio for roots as well. Increasing availability over the last decade of well washed and well buffered coco fiber has spurred continued growth in its use as a hydroponic medium.
Perlite
Perlite is a common potting soil amendment that can also be used as a standalone hydroponic growing media. Perlite is hydrated obsidian ore that has been super-heated and popped like popcorn in a furnace. Perlite is lightweight with great oxygen retention and for that reason makes a great soil amendment, but perlite can be more trouble than it’s worth to use as a standalone hydroponic medium.
Clay pellets
These have been around for ages. Hydroton Expanded clay pellets are exactly that. Expand-ed clay that forms porous balls that make a great hydroponic growing media. They have been used for years primarily due to availability, and the fact that they are reusable and pH Neutral. However, clay pellets are also heavy and dry faster than many modern growers would prefer and are not nearly as common as they were 15 years ago.
Rockwool
Rockwool is essentially rock that has been melted and spun into long fibers, much like fiberglass. Rockwool offers some fantastic water retention and aeration for the root zone but it has some significant downsides as well. Notably, it can be unhealthy to work around as fibers can and often do become lodged in the skin or breathed in and lodged in the lungs. This, combined with questionable toxicity, has made rockwool gradually less popular over the years.