db4o
::: "In principle the db4o database file format is fully compatible between Java, .NET and Mono, no matter on which operating system db4o is run." more about
db4o <a href="http://www.db4o.com/">here</a>, and 
<a href="http://www.people4objects.org/">http://www.people4objects.org/</a> is 
worth to take a look as well.