There are lots of companies that give small discounts to tickets. Still worthwhile, but don't expect to save 30%. Cheshire named the big ones. When evaluating them, watch tax and shipping charges, there is a significant different in price based off who adds what at the end. If you have some time, sign up for the Mousesavers newsletter. the offer given to subscribers is usually one of the better ones.
If you see a significant discount on tickets, it probably isn't legit, or involves sitting through a timeshare presentation.