Vermiculite

Improves moisture retention when mixed with soil.
Helps with aeration of root systems, although if this is your primary goal in using a soil additive, you should instead choose perlite.
Because it is a sterile medium, it is great for starting seeds and for propagating cuttings. Using plenty of vermiculite, or even straight vermiculite, can prevent bacterial and fungal problems such as damping off and root rot. It is commonly used for seed germination and in seed germination mixes.
Improves drainage and lightens the soil in the garden, in raised beds, or in pots.