The only named examples so far are the Spiteful Sisters of Seven-Handed Sek, an order who devote themselves to contemplation and Good Works, including the running of a girls' school in Ankh-Morpork. Vestigial Virgins have also been referred to, but it is uncertain as to what role these ladies fulfil in the religious establishment.
One prominent member of the Council of Churches, Temples, Sacred Groves and Big Ominous Rocks is female, commands the wary respect of Hughnon Ridcully, and may be a nun herself. (Although the High Priestess of Anoia, who we meet in Going Postal and Making Money, is very definitely a Priestess and not a nun).
It is noted that in order to escape attention and blend in with his surrounding, Mr. Tulip once dressed as an Omnian nun, using this guise to get into the newspaper offices so as to menace William de Worde. So nuns must be common enough on the Discworld, and certainly in Ankh-Morpork, for them to be unremarked on the street.