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.
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)