Bitsy is a small, fast, embeddable, durable in-memory graph database that implements the Blueprints API. It is an ACID database that recovers cleanly from power failures and crashes. Designed for multi-threaded OLTP applications, Bitsy implements optimistic concurrency control and stores data in human-readable JSON-encoded text files. Common management tasks such as online backups can be done over JMX. The speed and simplicity of Bitsy relies on three design principles: No Seek, No Socket, NoSQL.
Neo4j is a graph database, a fully transactional database that stores data structured as graphs. A graph is a flexible data structure that allows for a more agile and rapid style of development. You can think of Neo4j as a high-performance graph engine with all the features of a mature and robust database. The programmer works with an object-oriented, flexible network structure rather than with strict and static tables, yet enjoys all the benefits of a fully transactional, enterprise-strength database. The community edition is GPLv3 licensed, while the advanced and enterprise editions are AGPLv3 licensed.
HyperGraphDB is a general purpose, extensible, portable, distributed, embeddable data storage mechanism. Designed specifically for artificial intelligence and semantic web projects, it can also be used as an embedded object-oriented database for projects of all sizes. It is a Java-based product built on top of the Berkeley DB storage library. It can be used as a single in-process database bound to a location on the local disk or within a "cloud" of networked database instances communicating and sharing data in a P2P (peer-to-peer) fashion. Key features include storage of generalized hypergraphs, an open, extensible type system, basic query system and graph traversal algorithms, out-of-the-box support for Java object storage, thread-safe transactions, and a P2P framework for data distribution.