Gaffe is a project mainly written in Scala, it's free.
Graph database?
gaffe may eventually become a real graph database, but since it is scratching an itch caused by curiosity, rather than necessity, it may never go anywhere.
So far, the most interesting portion is the planned persistence model inspired by log structured merge trees and CouchDB's append only B-Tree, but with heuristics to layout optimize the inherent indexes embedded in graphs.
Alright, enough buzzwords.
Use sbt and locally published (for Scala 2.8) copies of
...to build gaffe.