Some plants contain high amounts of saponins and these are basically soap forming compounds, the higher the level of saponins the more 'soap' the plant will produce. This can be handy for basic hygiene when out in the Bush.
Two of the more commonly found ones are horse chestnut and soapwort.
Horse chestnuts ( we call them conkers) contain a lot of saponins, crush them up, and add to soft water and leave for 24 hours, the result is a good soapy liquid
Soapwort, as the name suggests is good too,
It aslo produces plenty of suds but to be honest, horse chestnut is better, again discard after 3 days or it begins to smell like mushy peas!