Drools - Drools introduces the Business Logic integration Platform which provides a unified and integrated platform for Rules, Workflow and Event Processing.
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.
Evrete - Evrete is a lightweight, open-source Java Rule Engine that is compliant with Java Rule Engine Specification (JSR-94)
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.
Ruote - Ruote is a workflow engine written in Ruby. Ruote is not a state machine library.
Vroom - Vehicle Routing Open-source Optimization Machine