From the experts:
Sally is a cheerful, fresh-faced girl-next-door name that was originally a nickname for Sarah, but has long been used independently. Sally was popular in the eighteenth century and then again from the 1920s to the 1960s--it was just outside the Top 50 around 1940. Though it hasn't been heard as a baby name for decades, we can see Sally bouncing back, especially after her exposure as young Ms. Draper on Mad Men--the Nameberries rank it at Number 621, and it's a Top 100 name in Sweden.
Sally has a ton of notable references--a few we can single out are astronaut Sally Ride, actress Sally Field, the Sally Bowles character in Cabaret, the movie When Harry Met Sally and songs like Long Tall Sally and Mustang Sally.