Higson.io - Hyperon is a BRMS, that was created with very large decisions and hyper-performance in mind. It stands out with the concept of the business domain which organizes the whole configuration in easy to manage way.
OptaPlanner - Mathematical optimization software
Decisimo - Decisimo - decision intelligence services and decisioning system. An engine for deployment of rule based and machine learning driven decisioning
Simple Rule Engine - DotNet simple BRMS XML rule engine
GoRules.io - Open-source business rules engine for automating decisions
Drools - Drools introduces the Business Logic integration Platform which provides a unified and integrated platform for Rules, Workflow and Event Processing.