Dobbin. Somewhat similar to ZODB, but much simpler. It looks nice but I haven't tried it myself. I haven't been able to find much information about it. It has been at version 0.2 for over a year. I don't know if it is still being actively maintained, if it is reliable enough for mission-critical applications.
From the description on the site, the main difference between Dobbin and other object databases is that persisted objects are read-only unless they are explicitly "checked out". This may make dealing with objects a little trickier, but it may make the database faster and more memory-efficient.
