Carnival (which owns Princess, P&O, Cunard, Holland, and others) as well as Royal Carribean (and potentially others) have a 'shareholder' benefit program. Basically, if you own 100 shares of their stock, they will give you an on-board ship credit. (for a 7 day cruise, it is like $100 per stateroom).

Check out their respective annual reports for the info. as well as the instructions on how to claim this credit. (Note: This is on-top of any discounts that you get when you book the fare. You claim this credit AFTER you have made a confirmed reservation).