Simple Workflow is a Java workflow engine that is really simple to use.

Features:

• Simple to use (need to call only 2 methods and implement 1 interface)
• Written in Java 7
• Uses a simple domain language to describe workflows
• Can be used with any storage backend (Sql, NoSql, File, etc.)
• Can be used with any frontend
• High level of backend/frontend abstraction

Simple Workflow is free and available  here 

There's already enough open source bpm/workflow engines in the java space. Plus, as the market has shown, there's no commercial market for stand alone BPM. Just look at all the BPM products that have failed to deliver the promise of easy workflow.

The BPM market has consolidated a lot the last 4 years. The only consistent market for BPM that I see is in vertical apps.

This is not as simple as it seems. The Java code is simple :) but defining the actual workflow seems too cryptic. Why not go for a more streamline XML definition?