Blazegraph (formerly known as BigData) is a graph database with support for the Blueprints and RDF/SPARQL APIs[1]. Blueprints is analogous to the JDBC, but for graph databases[2]. SPARQL is a query language for RDF[3].

Blazegraph is freely available under the GPLv2 open-source license[4]. Commercial licenses are also available.

One of the more touted features of Blazegraph is that "it supports up to 50 Billion edges on a single machine and has a High Availability and Scale-out architecture."[1]

tamsin woo has written an Ruby gem for connecting with Blazegraph:

Cornell has a nice document on how to get started with Blazegraph:

I've posted my first examples using Blazegraph standalone on this GitHub repo: