MiniZinc - MiniZinc is a free and open-source constraint modeling language.
Camunda - The Universal Process Orchestrator
OptaPlanner - Mathematical optimization software
jBPM - jBPM is a flexible Business Process Management (BPM) Suite.
Drools - Drools introduces the Business Logic integration Platform which provides a unified and integrated platform for Rules, Workflow and Event Processing.
ProcessMaker - ProcessMaker is an easy to use BPM and workflow software solution. It is used to design, automate, and deploy business processes of any kind.