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), Grow subscription ($43/mo), or Premium subscription ($84/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.