I planted Soapwort (Saponaria officinalis) in the garden a year or two back and it's slowly spreading and sprawling. Not a tidy plant but that doesn't matter in a 'wild' garden.
 Centuries ago Soapwort migrated across Europe from the Middle East. The leaves contain saponins and can be boiled to make a mild detergent. The wool and cloth trades of their day made use of this property so it was probably propagated as a crop of sorts. A simple soap can also be made by rubbing the leaf between the hands with a splash of water which results in a thin (and very green!) lather.