There are two possibilities as of 2012.
The first is Yowane Haku and is normally the character most are asking about when they ask for Vocaloids with white hair. However, she is a Voyakiloid/Fan made who is variant of Hatsune Miku who represents poorly made song and Vocaloid users who whine that people do not listen to their music. A artist Japanese artist, Caffein, was the one to originally illustrated her.
The only official Vocaloid with white hair is the Japanese male Vocaloid called Utatane Piko.
But either than that it could also be the 'Engloid' Miriam, since in most of her depictions she is described as a white-silver haired woman.
- See also