1. Apache Hama is currently focused on implementing pure BSP computing engine on top of Hadoop and its performance. They also plan to add a Pregel-like graph computing framework.
2. Y!'s giraph is different in the sense that it runs as a map-only job.
See the org.apache.giraph.graph.GiraphJob.BspMapper. They uses existing Hadoop Map/Reduce.
3. JPregel is a prototype level project I think. But, well written code.
4. And, GoldenOrb. Sorry, I didn't read their code.