Perlite is highly-absorbent expanded volcanic glass that works well as both a media additive to improve aeration, and as a growing media on it's own.
Inorganic, inert and sterile, it doesn't become soggy. It also insulates well against temperature fluctuations and makes for a popular 50/50 mix with vermiculite.
Available in 10l or 100l (UK mainland only) bags.