Nothing... it's free! Rafflecopter provides a basic (and awesome!) service free of charge. Many folks that use Rafflecopter are registered with a free account.
To make it even awesome-r, advanced features are available, set up on a monthly basic subscription ($13/mo), premium subscription ($84/mo), or enterprise subscription ($245/mo). Check out our pricing page for additional info or read our article here on our knowledge base that breaks down free and premium features.