Glycerin

A very common humectant moisturizer, usually seen as a base for or one of the first few ingredients in moisturizers.

Pure glycerin is said to absorb it’s own weight in water over a period of 3 days, and has been shown in numerous studies to improve skin elasticity, moisture, and skin barrier function. Pure glycerin is available to buy online and some people are known to simply mix a few drops into their favorite moisturizers and serums to boost its moisturization (although pure glycerin is very, very sticky and doesn’t feel great).

Glycerol accelerates recovery of barrier function in vivo 
Fluhr JW1, Gloor M, Lehmann L, Lazzerini S, Distante F, Berardesca E.
Acta Dermato Venereologica, November 1999

Glycerol replacement corrects defective skin hydration, elasticity, and barrier function in aquaporin-3-deficient mice 
Mariko Hara, A. S. Verkman.
Proceedings of the National Academy of Sciences of the United States of America, May/June 2003