I'm at the beginning of a research cycle on graph databases, and am curious to get some suggestions of your favorite open source projects? Mostly interested in extremely scaleable applications, for large scale processing of entities and relationships. And no this is not a joke I do not work for the NSA! Ha
Pretty much the same here. I am not an expert on any of these projects yet, but I can share some of what I've found and been researching so far.
and am curious to get some suggestions of your favorite open source projects?
Depends on exactly what you are looking for. There are graph "databases" and there are graph "processing engines" and there are subtle differences between them. Some of these things gain massive scalability by running on top of Hadoop, others implement Bulk Synchronous Parallel processing, etc.
A few interesting choices you could look at include:
Apache Giraph - http://giraph.apache.org/
Titan - http://thinkaurelius.github.io/titan/
Faunus - http://thinkaurelius.github.io/faunus/
Apache Hama - http://hama.apache.org/ - not strictly a graph database OR a graph analytics engine, but a generic BSP platform, written in Java, that works with HDFS. Does include some built in support for Graph processing: http://hama.apache.org/hama_graph_tutorial.html
Hypergraphdb - http://www.hypergraphdb.org/index
OrientDB - http://www.orientdb.org/
GoldenORB - http://goldenorbos.org/
You may also be interested in Blueprints, which is more of "just an API" but is supported by several of the various graph database projects out there. https://github.com/tinkerpop/blueprints/wiki
For what it's worth, I'm leaning towards focusing most of my initial energy on Giraph and/or Hama, but I'm also pretty excited about Titan/Faunus and OrientDB as well. I haven't read much about GoldenORB or HypergraphDB yet.
This blog most may also be of interest: http://nosql.mypopescu.com/post/40759505554/a-comparison-of-...
as may this one:
http://xinhstechblog.blogspot.com/2012/12/scalable-graph-pro...
The Google Pregel paper is also worth a read: http://portal.acm.org/citation.cfm?id=1807167.1807184