OpenWFE is an open source workflow engine. It is implemented in Java, and is available under the BSD license.
OpenWFE not only features a workflow engine, but also a full fledged Business Process Management Suite. OpenWFE contains these features and many more:
a worklist component for storing workitems (tasks) for participants,
an APRE component, allowing you to implement automated agents into your work flows,
Droflo, our web-based flow designer,
and a well-documented REST interface and several libraries to access it, with more on the way.
OpenWFE can run completely standalone without any Application Server, requiring only a Java SDK (1.4.x) or better.