I use db4o (GPL) within some projects. It is a really great peace of software. If you don’t need a relational database (and often you don’t!), db4o is definitly worth taking a look at it.
It is possible to use db4o with a client-server architecture. Often this is a faster and easier way to use db4o than some sort of RMI. Applications that shall use a common database but run on different machines (rich clients within a LAN) can benefit from those features of db4o.
For the next version (6.4) the performance of client server architectures is within focus - with success:
client server performace
http://blog.cedarsoft.eu/2007/09/22/db4o-speed-improvements/