Paxos
Paxos is the classical crash-fault-tolerant consensus algorithm introduced by Leslie Lamport in 1989. Paxos and its derivatives (Multi-Paxos, Cheap Paxos, Vertical Paxos, EPaxos, Flexible Paxos) are foundational to distributed systems theory and are used in Google Chubby, Spanner, Megastore, and others.