Activiti is a light-weight workflow and Business Process Management (BPM) Platform targeted at business people, developers and system admins. Its core is a super-fast and rock-solid BPMN 2 process engine for Java. It's open-source and distributed under the Apache license. Activiti runs in any Java application, on a server, on a cluster or in the cloud. It integrates perfectly with Spring, it is extremely lightweight and based on simple concepts.
ProcessMaker is commercial open source workflow management software designed for small to medium-sized businesses and organizations. ProcessMaker allows a business user with zero programming experience to model, automate, and manage a company's unique business processes.
ProcessMaker is a complete workflow management suite with tools to design forms, create documents, assign roles and users, create routing rules, interconnect with third party systems, and map a process quickly and easily. You can manage workflow from your favorite web browser without having to download any client software at all.
We believe in no vendor lock-in and source code visibility. Thanks to open source code, ProcessMaker integrates seamlessly with your organization's existing systems. ProcessMaker's light yet powerful workflow engine is simple to use and easy to extend.
L. Cabral, B. Norton, and J. Domingue. Proceedings of the 4th International Workshop on Semantic Business Process Management, page 9--16. New York, NY, USA, ACM, (2009)