Rosamond; mistress of Henry II, known as "Fair Rosamond" and quite possibly mother of William Longespee, Earl of Salisbury. [Burke's Peerage]
Note: Rosamond, although Henry's "favorite mistress", was probably not William's mother. A "Countess Ida", in William's own words in two different donations in her name, was probably his mother. Some think that Ida was connected with the de Toeni family. I await further developments before changing William's mother.