Bossa is a workflow engine written in Java. The engine is very fast and lightweight, uses a very expressive Petri net notation to define workflows, does not require a RDBMS and is very simple to use and to integrate with java applications.
Actually, it was designed to be embedded. Therefore, Bossa is organized as a library to be used by server side applications (especially web oriented ones) that need workflow services. We plan, in the future, to implement a shell over the current library to offer workflow services as a stand alone server.