db4o is designed to provide a full-featured, embeddable database engine
for equipment, mobile, desktop, and  server platforms in object-oriented
environments. Where relational databases fall short in providing zero
administration, small footprint, high performance, smooth synchronisation,
and easy refactorability, db4o is the answer. Native to Java and .NET,
db4o's single programming library (.jar  /.dll) easily integrates into the
application and performs highly reliable and scalable persistence tasks with
just one single line of code, no matter how complex object structures may
be.

WWW: http://www.db4o.com/