Most people tend to assume that fancy braid styles can only be done with extra long hair, but there are a number of braid styles that can be done in hair that ends at or above shoulder length. A lot depends on the texture of the hair. Fine, unlayered hair generally works best.
Many of the same guidelines go for braiding short hair as for braiding children's hair. Three strand braids will stay in best and look neater. Classic braids and crown braids are particularly good for when there isn't much length to work with. Cornrows are a good short style, but they require very nimble fingers. If the short hair is slightly thick, doing a half style will probably work better.
For more pictures of braids I've done with shorter hair, see my Flickr tagged set here.