Workflow Engine is a lightweight all-in-one component that enables you to add custom executable workflows of any complexity to any .NET or Java software, be it your own creation or a third-party solution, with minimal changes to existing code.
WorkflowEngine.NET - component that adds workflow in your application. It can be fully integrated into your application, or be in the form of a specific service (such as a web service).
Developer focused code only open source workflow engine developed by Uber. Highly scalable (100s of millions of open workflows).