Trinity Graph Engine!

You can think of Neo4j as a high-performance graph engine with all the features of a mature and robust database.

ArangoDB database is an open-source NoSQL solution with a flexible data model for documents, graphs, and key-values.

The Document-Graph database, SQL and Native Queries, asynchronous commands, intents, and much more.

FlockDB is a distributed graph database for storing adjancency lists, with goals of supporting:

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.

Wikibase is the software that runs Wikidata, but is also usable for other projects beyond that. Wikibase allows the collaborative editing of structured data on top of MediaWiki. It is optimized for graph-like, schema-poor data, not for tabular and strongly pre-structured data.

Unlike traditional relational databases or Property Graph Databases, AllegroGraph employs semantic graph technologies that process data with contextual and conceptual intelligence. AllegroGraph is able run queries of unprecedented complexity to support predictive analytics that help organizations make more informed, real-time decisions. AllegroGraph is the first Graph Database to support analysis across N-dimensions - any conceivable measurement of an object, property or operation. For example, AllegroGraph can analyze temporal (time) and geospatial (location) dimensions relative to any 'event,' such as a disease, drug interaction, genetic combination, biomarkers, observations, image or physical sensors. »

Bitsy is a small, fast, embeddable, durable in-memory graph database that implements the Blueprints API. It supports ACID transactions with optimistic concurrency control and on-disk persistence.

NetworkX is a Python language software package for the creation, manipulation, and study of the structure, dynamics, and functions of complex networks.