Soapwort (Saponaria officinalis) is one of those plants that wanders from where it was planted by way of rambling rhizomes. Not one for the neat herbaceous border but I introduced it to the garden because of its spreading habit.
 The leaves do indeed lather up to make a thin soap when rubbed between the hands with a splash of water. Historically the mild detergent it produces was used by the textiles industry and Soapwort was grown as a crop to be harvested for that purpose.