The famous or rather infamous SLS (not to be confused with SLES). It is a cleansing agent known for being too good at the job and potentially irritating the skin. But, on the positive side, it can produce copious, creamy and luxurious foam compared to the more gentle and thus nowadays much more commonly used Sodium Laureth Sulfate.
In fact, SLS is so good at irritating the skin that it is very commonly used in dermatological studies just for that. It is a so-called "primary irritant", a substance that irritates the skin in one go (without prior sensitization) but doesn't do any other big harm (such as being carcinogenic or systematically toxic - those claims are not true). Also, the formula can greatly influence the irritating potential of SLS, and mixing it with other cleaning agents makes it milder.
If it's not in a cleanser, it works as an emulsifier or even as a penetration enhancer for active materials.
Show me some proof
- Lee, Cheol Heon, and Howard I. Maibach. "The sodium lauryl sulfate model: an overview." Contact Dermatitis 33.1 (1995): 1-7.