There are several plant or vegetable based sudsing agents out there, unfortunately most manufacturers like to keep the exact ingredients to themselves :) and it is important to note that their formulations while plant/vegetable based might not be completely "natural" which is what this question asks for. Also, since this is the cosmetic section, I assume you are refering to agents used in cosmetic and beauty products. This is one that I know of (www.a-vital-life.com/page.php?p=glossary) Shikakai extract (Acacia concinna) - Used in place of soaps and harsh detergents, this water soluble extract, which serves as the primary cleansing and sudsing agent, is made from shikakai powder from India. Shikakai (Acacia Concinna) is a small shrub-like tree which grows in the warm, dry plains of central India. For centuries the people who have had access to this tree have used its pod-like fruit to clean their hair. They collect, dry and grind this pod into a powder which is considered a superior cleanser. Today this fruit is grown commercially in large quantities in India and the Far East. As a result, shikakai powder is readily available and continues to be commonly used as a preferred shampoo. Typically, shikakai is mixed with water to make a paste which is worked through the hair. It lathers moderately and cleans hair beautifully. It has a natural low pH, is extremely mild, and doesn't strip hair of natural oils.

Shikaki (Acacia concinna), Sapindus mukorossi (Soapnut tree)are the plants which are used as natural surfactants, they are natural, harmless, low pH, mild and very useful as medicine.