From Oxxus Wiki
Is an object database for Java and NET developers aimed to simplify code embedding for operations over databases.
The db4o is written in Java and NET and provides respective API's.
Db4o Database can run under any system that has Java or NET capabilities thus offered under several license including the GNU General Public License (GPL), the db4o Opensource Compatibility License (dOCL), and a commercial license for use in proprietary software.
Some main features of Db4o
One-line-of-code database which offers ability to store any object within single line.
Embedding feature that provides complete integration within clients or other software components.
Client-server version allows Db4o to communicate between client and server-side applications but has certain limitations and is highly dependable on network bandwidth that server is attached to.
Dynamic schema evolution offers automatic schema evolution for basic class mode change while more complex ones like field name or field type are not automated.
Native Queries provides ability to use programming directives of programming language from which db4o is used. There's no need for string-based APIs (such as SQL, OQL, JDOQL, EJB QL, and SODA).
Portability and Cross-platform Deployment
Db4o supports Java's JDK 1.1.x through 6.0 and runs on Java EE and Java SE. Db4o also runs with Java ME dialects that support reflection, such as CDC, Personal Profile, Symbian OS, SavaJe and Zaurus. Depending on customer demand, db4o will also run on dialects without reflection, such as CLDC, MIDP, BlackBerry and Palm OS.