Stellar is a open source consensus protocol, forked out of Ripple.
White paper by David Mazières from Stellar Development Foundation “introduces a new model for consensus called federated Byzantine agreement (FBA). (…) and Stellar Consensus Protocol (SCP), a construction for FBA. Like all Byzantine agreement protocols, SCP makes no assumptions about the rational behavior of attackers. Unlike prior Byzantine agreement models, which presuppose a unanimously accepted membership list, SCP enjoys open membership that promotes organic network growth. Compared to decentralized proof of-work and proof-of-stake schemes, SCP has modest computing and financial requirements, lowering the barrier to entry and potentially opening up financial systems to new participants”