Diana Wynne Jones - a lot of her stuff is out of print but you can find it in good children's libraries and used on amazon. The Chrestomanci series is very enjoyable, and I loved loved loved The power of Three.
Pippi Longstocking has already been mentioned, but for more serious fantasy books by astrid Lindgren, check out Ronya the robbers daughter, the brothers lionhearted and Mio my son.