Blockchain-based systems are enjoying unbroken popularity. Different economic and social actors are investigating their application for fostering decentralization and separation of power. Whether a blockchain-based system can live up to such goals is heavily determined by the choice of a consensus protocol – the rules by which participants agree on what gets added to the blockchain. Bitcoin’s consensus protocol is inherently decentralization-enabling, at a notoriously high ecological cost. So-called permissioned protocols, while incomparably more efficient, are dismissed as being closed-off and "centralized". Federated blockchain systems represent a middle ground between these two extremes and promise to offer openness and security without sacrificing ecological sustainability. As a rough approximation, their approach can be described as bootstrapping consensus from a web of trust. In this overview article, after a short review of the Bitcoin approach and possible alternatives to it, we introduce the ideas behind federated blockchain systems and discuss their impact on future blockchain systems.