I have tried the Object Manager and the Boo browser, but none of these work with the latest DB4O file format.
The Object Manager which comes with db4o itself should be able to view the database-files. Otherwise it's a but a you should report it.
The Object Manager which comes with db4o is a Eclipse-plugin for the Java db4o version and a Visual Studio Plugin for the .NET version.
In the Java-Version it is in the ome directory of the db4o distribution. You can unzip the Object Manager. Then add that directory as a Eclipse update-site and install the Object Manager.
For the .NET Version there's a omnomn2005, omn2008 and omn2010 folder. Pick the version acording to the Visual Studio version and install it.