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.
Decisimo - Decisimo - decision intelligence services and decisioning system. An engine for deployment of rule based and machine learning driven decisioning
OptaPlanner - Mathematical optimization software
GoRules.io - Open-source business rules engine for automating decisions
DecisionRules.io - Business rule engine that lets you create and deploy business rules, while all your rules run in a secure and scalable cloud. Unlike other rule engines, you can create your first rule in 5 minutes and make 100k decisions in a minute via API.
MiniZinc - MiniZinc is a free and open-source constraint modeling language.