Shirley Henderson

Sometimes I think that this double-guest star might be my favorite for her unexpectedness...
I first knew her as "Moaning Myrtle," but she has quite a list of credits- just check out her IMDb listing:
http://www.imdb.com/name/nm0376602/

movies.about.com

Harry Potter's ghost girl:

freewebs.com
Dr Who's Love and Monsters episode, Ursula Blake

tvguide.com

tardis.wikia.com