If you haven't at least looked at this product, you should! Among some of it's features is the ability to save data with one line of code, object oriented(!!!), embeddable(only a 350K footprint!), and multi- and cross- platform support.
Excerpt from the press release:
Additional improvements in db4o Version 4.5 include a pluggable reflector and a new generic reflector that help developers build more diverse and exciting architectures, along with a new object-oriented ObjectManager for easy browsing of database files.
db4o's pluggable reflector interface allows Java and .NET developers to write their own reflectors. Version 4.5 advances this functionality with a powerful new generic reflector further enabling tuning for distributed systems, cross-platform support, and enhanced encryption capabilities.
With the generic reflector, developers can now run a db4o server without having to deploy application classes. They can also easily access objects and values where classes or fields are no longer available as source code.
One implementation of this is the new ObjectManager, which allows developers to "look" into databases and monitor what's happening in real-time while they write applications. It can also be used by third parties to browse a database file, and/or to build ad-hoc queries for instances of a class. This helps ease the transition to OOP-thinking for developers new to OOP or ODBMS or both and boosts the benefits of OOPs such as Java and .NET.