Titan is a scalable graph database optimized for storing and querying graphs containing hundreds of billions of vertices and edges distributed across a multi-machine cluster. Titan is a transactional database that can support thousands of concurrent users executing complex graph traversals in real time - thinkaurelius.com 
Open source with the liberal Apache 2 license.
* Download Titan - github
* Clone from GitHub
* Read the Titan documentation - thinkaurelius.com
* Join the mailing list - groups.google.com
# Alternatives
* Arangodb (C++ Apache 2 license) - arangodb.com
* GraphChi (C++ Apache 2) - github - technologyreview.com
# Features Titan provides the following features:
* Elastic and linear scalability * Data distribution and replication * Multi-datacenter high availability and hot backups * Support for ACID and eventual consistency
Support for various storage backends: * Apache Cassandra * Apache HBase * Oracle BerkeleyDB
Support for global graph data analytics, reporting, and ETL through integration with big data platforms: * Apache Spark * Apache Giraph * Apache Hadoop
Support for geo, numeric range, and full-text search via: * ElasticSearch * Solr * Lucene
Native integration with the TinkerPop graph stack: * Gremlin graph query language * Gremlin graph server * Gremlin applications
# Combined with SimpleDB
One of the main benefits for us is that AWS has provided backend integration with DynamoDB. This affords you practically infinite and painless scaling on a pay-as-you-go model - aws.amazon.com
# See also