Db4o is an object database, i.e., forget about the mapping of tables in a relational database model. If you're a developer, that translates into savings in time invested in your application and volume of code. Db4o's great potential is that you can reuse your [plain, non-mapped] objects by saving and retrieving them as many times as you want. You can persist complex objects or design patterns with nested collections or other complex objects at any level of complexity in your hierarchy. You're not limited to flat objects with primitive types to get reasonable performance.
With db4o you can also benefit from native queries which bring you closer to the language and combine optimized execution with simplicity even for complex queries. Db4o is open source and can be downloaded here.