Graphbase is a project mainly written in ..., it's free.
Implementation of a graph db on top of HBase
Graphbase is an implementation of the Tinkerpop Blueprints APIs on top of HBase. For the moment it's an experiment to see how the bigtable model is suitable for storing a graph. I'm thinking to implement some M/R algorithm on top of this infrastructure and I'm thinking to add a Scala based DSL. More to come soon ...