Saponaria

Saponaria is a goddess of soap and cleanliness. It may be assumed that practices associated with her involve soap as it is noted in The Complete Ankh-Morpork the use of soap around the holy water fonts in the Temple of Small Gods is prohibited except among followers of Saponaria, while Lord Vetinari mentions in The Science of Discworld IV: Judgement Day that she arranges 'wonderfully fine, smooth and plentiful' suds.

Annotation

Saponaria is the name of a genus of plants, in the Caryophyllaceae (Pinks and Carnations) family. This family - and this genus in particular - are notable for their Saponin content (named for the plant), which is responsible for creating a bubbly lather from the leaves and roots, useful as a gentle soap. It's names reflect this: "Sapo" is derived from Latin meaning "soap", while a common name for Saponaria officinalis is "Soapwort" ("wort" meaning herb).