eth-gas-refund
This is a simple mechanism in Ethereum to refund the gas the user had to spend interacting with one's smart contract. This is one way to incentivize users to interact with a new dapp.
With this mechanism, if the smart contract author did not want to risk losing funds, the assumption is the smart contract would earn more from the user's interaction than the cost of gas.
An extension to this can be to limit the gas refund to a limited number of interactions per Ethereum address so that new users are subsidized but regular users are not.