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.
VoltDB is a blazingly fast relational database system. It is specifically designed to run on modern scale-out architectures: fast, inexpensive servers connected via high-speed data networks. It is aimed at a new generation of database applications - real-time feeds, sensor-driven data streams, micro-transactions, low-latency trading systems - requiring database throughput that can reach millions of operations per second. What’s more, the applications that use this data must scale on demand, provide flawless fault tolerance, and enable real-time visibility into the data that drives business value. It includes client application drivers for applications written in Java, C++, C#, PHP, and Python. VoltDB community members have also authored client libraries for Erlang, Ruby and Node.js. There are streaming export capabilities for leading analytic database environments, including Apache Hadoop.