Twitter recently released an open source large scale graph precessing library named Cassovary. The library is written in Scala and therefore can be easily integrated into any JVM-based language. The library focus is on simplicity and efficient data structures to support processing of large graphical structures.
From the project README:
Cassovary is a simple “big graph” processing library for the JVM. Most JVM-hosted graph libraries are flexible but not space efficient. Cassovary is designed from the ground up to first be able to efficiently handle graphs with billions of nodes and edges. A typical example usage is to do large scale graph mining and analysis of a big network.
The library appears to mainly provide data structures and some common graph search algorithms. Any dataming or graph related operations are currently not part of the library. It looks interesting and should generate a lot of interest.