db4o (database for objects) is an embeddable open source object database for Java and .NET developers. It is developed, commercially licensed and supported by Versant.
db4o is written in Java and .NET and provides the respective APIs. db4o can run on any operating system that supports Java or .NET. db4o is offered under multiple licenses, including the GNU General Public License (GPL), the db4o Opensource Compatibility License (dOCL), and a commercial license for use in proprietary software.
Read more about Db4o: History, Place in The Database World, Disadvantages, Portability and Cross-platform Deployment, Documentation and Support, Object Manager, Community, Development Process, Versions