Neo4j is an open-source, NoSQL, native graph database that provides an ACID-compliant transactional backend for your applications that has been publicly available since 2007.
Neo4j is offered as a managed service via AuraDB. But you can also run Neo4j yourself with either Community Edition or Enterprise Edition. The Enterprise Edition includes all that Community Edition has to offer, plus extra enterprise requirements such as backups, clustering, and failover abilities. Neo4j is written in Java and Scala, and the source code is available on GitHub.