My sister has donated her long hair to an organization called Locks of Love. From what I could find online, this organization uses the donated hair to make wigs for children who have cancer and are losing their natural hair because of the chemo treatments that they must undergo.
There is also a program sponsored by the hair products company, Pantene, called Pantene Beautiful Lengths. Their program uses the donated hair to help women who are fighting cancer.
I think both organizations are worth checking out they have good web sites with information on how long the hair must be, where to send it, etc.