The Mountbatten Institute organise year-long internships in business in New York or London through an educational programme. The majority of interns are placed in finance, law, non-profit and marketing. As well as a corporate placement, Mountbatten provide housing, lectures and organisational support.
They run two annual cycles starting in March and August. For the 2011 cycle there were approximately 350 interns in New York, providing an active social network.

You can go to their website here or questions of current interns on forums here and here[links open in new page].